They keep honest people from playing the game, unless you have clean install of windows with no blacklisted drivers or software installed. Not to mention how these things basically hook themselves to critical system APIs, acting more like a malware. Valorant is probably worst example of this. Community run servers are the best form of "anti-cheat".
The problem is that developers treat PC like a locked console. This is just a completely fruitless uphill battle. PC gives power to its users, while consoles give power to the developers. PCs are designed to not sandbox or lock you, you can do anything with them without having to break its sandbox first. The mindset with the developers that deploy intrusive anti-cheat is to have the users locked in so they can ship their centralized server model and hope they can deal with the hopefully lesser amount of cheaters themselves, instead of giving the moderation power to the users themselves.