First of all you have to read the product description for the Grand Theft Auto V iFruit App on iTunes:
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. Maybe not. At least you’ll have something to keep you entertained while your significant other prattles on endlessly about their day.
Nice. So what can you do with this app? One part of it allows you to create custom cars with the Los Santos Customs app. That, and you can create custom license plates for Grand Theft Auto Online.
Then there's the Chop the Dog app. You can look after Chop by feeding him, playing games, and teaching him tricks. Take care of Chop successfully and Chop in the main game will be more helpful to Franklin, sniff out hidden items, and more.
A silly app, but I do like that it actually has some small benefits to you in the core game. You can grab the iFruit app for free right now on iTunes.