Anti-cheat. Knowing that if you get into a match with other people they won't be running software on their system that gives them an unfair advantage.
??? It's not the early 1990s anymore where you had to fiddle with autoexec.bat and assigning IRQs to get your DOS games to run. The only "downside" of PC gaming is that the hardware isn't subsidized like game consoles are, so you're upfront investment is a couple hundred bucks more. But then you don't need a subscription for multiplayer gaming, and PC games are cheaper (80 Euros for a game? come on...)