Its for DRM. The easiest way to check if you actually own the game is to have the game contantly ask whether its connected to the server. The server should have your payment info. If thats not found, your game isnt legit.

Thats why GOG is so good, their games dont have DRM. Meaning you (and pirates) have a BETTER version of the game. Let me repeat that, downloading an illegal copy gives you a better running version of the same game you mightve paid for, because it doesnt have to contantly talk to a server before the game will allow you to do anything.

When you pirate a steam game, youre also downloading a “fake” version of steam, a steam emulator. The way they break DRM for steam games doesnt remove the DRM, but it slightly reduces the issues DRM causes because the server it is communicating with is local and doesnt have to index user files. Your game just asks the steam emulator if its legit, the steam emulator doesnt check anything, it just says that whatever got checked was legit.