Have you ever seen a player in DM who stands still and has a bad robotic aim? That's a case farm bot. They use a program that lets you use multiple accounts at the same time. Have you ever been instakicked from a DM when you join? Often a full lobby of bots.
TF2 has it as well for some reason, but the cases there costs 0.01 $.
It's basicly bitcoin mining, but with bigger yields.