It's all the rage for AAA console games to have smartphone apps that hook into the main console game. So, yeah, GTA V has one, too. But, at least Rockstar Games isn't taking theirs too seriously.
Here's how iFruit is described on the iTunes App Store:
"How would we survive without apps? Apps can transform your mobile device into essential tools like a harmonica, disco ball or convincing moustache. Somewhere out there is the perfect storm of touchscreen technology to fix everything that's wrong with your life. And maybe all you're missing is the iFruit app."
iFruit does have some actual usefulness, though. When you log in with a Rockstar Social Club account, the iOS app let you create bespoke licence plates and customise vehicles that you'll drive in the game. It also lets you train Chop, the Rottweiler belonging to playable character Franklin. By playing fetch, tug-of-war and helping him rip the bikinis off of Los Santos' sun-worshippers in iFruit, Chop will behave better and help sniff out hidden items in GTA V.