In series play, there is a rhythm that players get into, generally they have prepared a style to use and after a game or two, they have a good sense of how it interacts with their opponent's. In a world where no one cheeses, going into a 2nd/3rd/4th game or whatever, the most common place where their brain will be is deep in the little timings and adjustments they need to make to move that interaction in their favor more. The thing is, this is when someone is most vulnerable to some sort of cheese, because they are possibly thinking of how to cut corners, or instead of thinking "what's the danger now?" they are thinking about if they need to get some upgrade before they expand this time.
Example (SPOILER for GSL Ro32):
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The Little One and Hyperdub played two games in a row of similar openings pursued by each player, TLO playing an unorthodox thor opening that he seemed to have prepared for this match. Hyperdub made some adjustment in game 2 by going less for banshees and more for tanks and vikings earlier, but in both games Hyperdub ended up ahead after the early/early-mid game.
At this point, TLO has to be thinking in his build, what steps can I take to not end up behind again in game 3. At this point, Hyperdub makes a very savvy play, saying "I'm betting at this point, he's not concerning himself with safety enough, he's concerning himself with further down the line strategy." And he bet correctly, as TLO did not play safe (scouting his own natural/scouting close spawn first).
There is also the consideration that after a string of games without any sort of cheese, players can just be lulled into forgetting that it can happen to them, especially when the overall strategic trends of a series have been somewhat established.
The next benefit of cheese is being able to affect your opponent's mindset. Think of Jaedong here. Players know they absolutely cannot ignore their early safety against him because he is ruthless (Stork vs. Jaedong in OSL semis for one of many references). What this does is it disrupts their flow, prohibits them from cutting corners without seriously outsmarting him (hi Flash), and puts them under mental pressure from the outset, makes them scout early and pay closer attention which cuts into the attention they can give to laying out a super complex long term plan.
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Did anyone notice how just by being known as a cheesy player DAVIT affected Clide's play, making him spend more time scouting the map. Now it didn't end well for DAVIT, but imagine he's at Clide's level but has the same cheese ability. He gains an advantage just from reputation, as Clide must pay a bit more attention in the early game.
For players who don't cheese very often, it can be very powerful as well. No one expects IdrA to 6 pool, so I'm sure he could catch some protoss totally off guard in a big match with it at some point.
All in all, knowing the right time to apply a cheese in a series is a very important skill that's quite misunderstood, and it's clear to me that a player with no cheeses in his arsenal is at a clear disadvantage. For a tournament player, it's an essential facet to develop.