The difference between nerfing and buffing is the difference between addition and subtraction - the number either goes up or goes down. However, nerfing and buffing in game balance has one rule that will always be true - the value of something is never null. If it is, then it's not a value anymore. If a marine does 0 damage with his gun, he's not doing damage. If he does .00000000000000000000000000001 damage, he's still doing damage, but only at a rate that in the given balance would be considered useless, or null. But it is not in fact null because the other values could be brought to the same level where such minimalist damage would now be considered worth something to the user, and not null.
A value in a gamecan therefore only be relatively null and not completely null unless it is non-existent. When talking about game balance, it's important to note that the act of nerfing or buffing things in themselves carry no weight unless there are other things to compare to it. Neither are good choices simply for their sakes - i.e. buffing is always better than nerfing or vise versa. Nerfing something into oblivion is no different than buffing it to God status.
Competitive games like DotA, Starcraft, League, Counter-strike, Hearthstone etc. are their own eco-systems. I'm not even talking about the community surrounding the games either - the games themselves are ecosystems. While a developer may have created the game, it's impossible for said developer to understand the depth of a change they may make because once a game's complexity reaches a certain point, changing anything could have a wider spread effect than one sees, especially when you can only see from one vantage point clearly.
It's not just competitive games that re like this either - Earlier generations of games have had all sorts of flaws pointed out because the developer thought they had done a good enough job hiding exploits. One of my favorites is the Zelda: Ocarina of Time glitch where you kill Ganon with a deku stick - The deku stick has a constant hit box, but you can only use it as a gild. You have to fight Ganon as an adult, but a certain glitch in the first dungeon of the game allows you to fight him as a kid, making the fight absurdly easy because you can just stand behind him.
The point of the deku stick having a constant hitbox was so it could catch fire when it collided with any flame. This is something you do a lot in OoT where you light torches in a certain order or to reveal another secret. Nintendo at the time believed that because the Deku Stick could not be used as an adult, why should they care? They didn't know about the Gohma glitch, and if they did, the way it's done requires precise understanding of movement and position that it would be a freak accident for people to figure it out.
Eco-systems take time to adapt to new changes that are introduced to it. Video games (competitive and non-competitive) are not much different in that aspect. In the times of Ocarina of Time, we didn't have consoles hooked to the internet to patch bugs out and prevent people from using glitches to get places in games. Modern day games are provided with such conveniences, but at a consequence - similar to how the Zelda glitch above allows someone to fight the last boss with ease, changes via patches can cause similar effects. The delicate art of balance is not simply nerfing and buffing - it's also understanding when the eco-system has time to adapt to a new change, especially when it will likely have unintended consequences.