Map control is central to StarCraft:BroodWar. Most of the games people lose at the lower levels of StarCraft occur because of an error of judgement regarding who has map control. When watching StarCraft:BroodWar VODs, map control is central to understanding the weaknesses and strengths of pro players' strategies. More than that, understanding map control is what will enable one to make up logical strategies of his or her own.
A Standard Example of Low Level Misjudgements
Imagine it's midgame in a PvZ. It doesn't matter how many bases either player has, or what their unit compositions are. The Protoss moves out to attack, and the Zerg responds by sending his whole army to fight it. The Zerg loses his army, and the Protoss proceeds to kill an expo. Why did the Zerg lose?
If you aren't thinking about map control, and you just played this game, you're probably thinking 'damn it!' and rightly so. 'Why did I lose? Didn't I make the right units? Didn't I keep my money low? Was there a place I could have made more drones earlier?' The answer to all those questions is maybe. But I didn't give the details of the army for a reason. Let's assume Zerg did everything as well as his opponent up to that point in the game. Even if he didn't, it wouldn't affect the decision he is supposed to make in this scenario. The fact of the matter is that Protoss has map control at this point. That's a hard thing to deal with as a Zerg, cause you're thinking you have more expos so you should have control of the map, but it's the truth of the situation. When Protoss has map control, by very definition your army is not strong enough to beat his. If he attacks with his army you have to make a judgement about whether or not you can defend it just by delaying for more units, or if you need to build sunkens to stop it. This will depend on your build, but this is absolutely the only way for Zerg to stay in the game.
Let's think of another example. You've just killed a Zerg army. You're really happy about it, and you think 'well I killed his army, so what's stopping me from killing his expo LOL.' You're right, you should go make a move on his expo. But the logic of this move is not to kill his expo. The logic of this move is to force Zerg to spend a lot of money building sunkens and army. If you can kill it anyway, that's great, but it you can't, it will be the critical failure in your game. Let's say Zerg didn't lose his army, and responded correctly. He pulled back and Protoss rightfully realised he has map control. So Protoss moves out to threaten the Zerg expo, and expands himself in the mean time, because what the hell is Zerg gonna do about it while he has map control. But then Protoss loses all or most of his army attacking the Zerg expo in a misjudgement of army size. The reason this is a critical failure is because he has given up the map control he just gained. Zerg can then respond by playing the move Protoss just did, in other words, expanding while threatening the expo Protoss doesn't have an army to defend.
In really newbie games this cycle goes on endlessly. One player gets map control, then immediately gives it back to the other player by charging into cannons/sunkens/lurkers/storm whatever. When you start playing against better players, however, you get immediately punished for these moves. You think at first 'I'm just not macroing well enough' or 'I'm just not microing well enough,' but the critical flaw in strategy usually revolves around a misunderstanding of who has map control (or a total absence of consideration for it).
In Pro Matches
All pros in BroodWar understand very well who has map control exactly when and for how long. All pros understand that when fast expanding, they are necessarily offering map control to their opponent. What keeps the player from going crazy with their map control is the various tech paths the player without map control is threatening.
In ZvP Dark Templar are a constant, palpable threat for the Zerg. Expand too much and you can't protect your bases from a single unit. Dark Templar are a unit that threatens to take back map control from a Zerg who is too greedy. Even if the Protoss player doesn't build them, that threat is always guiding Zerg.
Remember what I said about map control shifting from one player to another throughout the course of a game? How do you think this applies to ZvP? When a Zerg is choosing his third expo, he knows that eventually he is going to lose map control. Eventually the Protoss is going to get enough Templar, enough Zealots, enough Dragoons off two base that he will simply not be able to stop the Protoss from moving out and taking his third. As must be common knowledge, Zerg don't like being on the same number of bases as Protoss. It's uncouth. So what does Zerg want in a map when expanding?
The answer, as you've seen in countless progames, is that the Zerg wants a third which can be used to protect his fourth. When the Zerg loses map control in the midgame, the most efficient way to keep the Protoss from crushing his expos is just to build a bunch of sunkens. But sunkens don't win back map control. They put you farther behind, as a matter of fact. The counter to this is if they can take their 4th expo without it being threatened by the Protoss, they're back ahead in expos and it's still difficult for toss to take a fourth.
I apologise if this is incredibly tedious, or obvious, but I think you'll find if you start thinking about map control throughout a game, instead of just as specific situations such as 'I have muta so Terran can't move out now' you will find your experience of BroodWar and watching BroodWar enriched. There are many cases of map control where build orders and standard tech switches have already been made. There are many cases where you want to know 'why does this work?' There are many times when you should know why an aggressive one-base strategy is so risky and why the defending player is not necessarily behind if he loses a lot defending it. It's not just economy. It's the map control that allows you to have that economy. It's the map control that gives you the ability to deceive your opponent. Protoss can't get a probe outside of his main? Suddenly the Zerg feels a lot more comfortable. Suddenly the Zerg doesn't have to worry about being surprised. Suddenly the Protoss does.
This is why if one players army is defending a base, the other's should usually be out on the map. If both players are out on the map, but battles aren't occurring, it's because one is avoiding the other. Threatening expos as the game gets more complex and map control is given to each player as percentages. I control the ground, so you can't attack me directly, but you control my vision, so your drops are threatening. I control the centre of the map because it is wide open, but you control any narrow area you can get to because I don't have dark swarm yet. This is how map control works, and this is the basis of all strategy.
For Some Reason
I feel like this is too obvious. But I've played BroodWar so long that it's become just a natural way of thinking about the game. And I know people still post replays of themselves asking 'why did I lose' when this is almost always the answer. Sometimes people lose map control because they are just too slow at teching, or their macro is bad, sure, but lots of times your opponent's wasn't much better. Some of map control is taught to newbies with Artosis' iconic phrase 'when you're ahead, get more ahead' but that leaves out so many important aspects of map control.
I remember long ago how enlightening, how wonderful it was to learn about map control in it's most fundamental ideas. Everyone knows Terran need vessels to move out against a lot of lurkers. Everyone knows mutas can keep Terran pinned into his base for a little while. But those are such specific situations, when the idea of map control is so much more general than that. Map control is a constantly present factor in BroodWar. If you have it, you need to know you have it so you can take advantage of it and expand. If you don't have it, you need to know that so you don't move out and lose all your shit. Make drones to make money. It's that fundamental to the game. It will raise your understanding of matches so much higher if you haven't grasped it yet.
In any case, I hope at least some people get something from this post. I'm not the best player by any means, but I do feel like I have a strong understanding of the game. If anything, even if you already understood it, maybe you'll think about it more the next few VODs that you watch ^^