Games

Modern cheats for multiplayer games don't modify local files (or attribute values in memory), since the server validates everything anyway. They're about giving you information that's available but not shown in the game (like see-through walls, or exact skill ranges), or manipulate input (dodge enemy damage, easy combos). Those cheat can run in kernel mode (or at least evade detection from user mode), so the anti-cheat needs kernel mode to be more effective.

They can prevent you from running cheats that other anti-cheats can't detect. For instance, they could modify the value in memory so that your calculated hash always succeeds even when it's modified. This doesn't stop cheating though; it just means cheaters have to use cheat hardware that exists at a layer that even kernel anti-cheat can't detect.

And then a game gets updated so the hashes don't match and uh oh, everything is fucked. Oh, but we can change the hashes of the files in the executable! Yeah, so can they. People modding shit into the executable is basically a given. Let alone the fact that you'd need to sit through a steam "validation of files" length of time every time you'd need to launch a game (because validation works exactly as you have described).

What is gained is that it has access to more information. Some cheats use an entirely different program / process that reads memory and outputs info that is available to the game but hidden from the player. Like a client needs to know where a person on the other team is to be able to draw their model. So you read that, you put a little box over where they are, and bang you have wallhacks.