Why do you restart your PC after installing a game

[RineEater]

I'm just curious about this, but why after installing a game do you have to restart your pc? Some games you don't have to do this and some you do. And if the game says the "Do you want to restart your pc now?" thing and I say no, will I be able to play the game without restarting it? Will it mess up the game or my pc?

 

[fullmetalvegan]

I'm not entirely sure why some games say that, probably if they altered DirectX files or think they did they will wanna restart. But no it's not necessary to restart in almost all cases, I never do when it tells me to. The worst thing that could happen is the game won't launch and you will have to restart, it won't damage any files.