Discs break, dics check = SecuRom (or even worse, StarForce) rootkit in your machine that you pretty much can't ever remove, and if you think those don't have these kind of launch issues, you've just been very lucky.
Installers don't work, or you install and then the game refuses to recognize your disc as valid, because of OS/driver/some app you installed/itunes changed the registry values of your optical drive/your motherboard's sata controller is weird/your optical drive wasnt tested/monkeys. The copy protection crashes the game. The de-registration tool for DLCs crashes, and when you reinstall your OS you can't play the game anymore. Its alllll the same all over, piracy protection has never worked on _any_ game, even ubisoft's, all it does is give legitimate owners grief, but with steam at least you're not bound to a disc that will eventually simply fall apart on you.
Also, some of us don't actually have optical drives anymore, whether by choice or because our laptops have them external.