DRM helps sell games in the way that storyline, or voice acting or graphics help to sell a game.
Good, helpful, non-intrusive DRM, that actually brings extra features along with it so you don't feel like they're just punishing the paying customers - Steam, I'm looking at you, enhances the games on that platform, and encourages people to place their games for sale there as they see there's protection in place, without punishing legit customers.
(Unless they're either batshit insane - see people who demand their own DRM on top of the Steam stuff, or greedy as all hell, EA pulling stuff to put on ORigin only.)
Crappy, game locking,half working, bug inducing, customer infuriating DRM however, that'd be a bad thing and not really a sales point.
I'm not anti all DRM, but there's gets to a point where you're spending a million bucks on security to protect something worth ten bucks, knowing there's an unblockable trapdoor that leads directly into the vault anyway.
You print a CD key, it gets typed in and authorised by a central server, they've just proved they bought it and it's legit, now step away and let them enjoy what they're paying you for.