You don't need to do it, but If you want to bring the best out of your mod get the tune.
It will make your mod worth it.
you can buy on iPhone and use it as an iPod Touch but until you get a cellphone plan it won't bring out the full potential of the iPhone..In other words get the Chip, and make sure the people you get it from know what they're doing and that they do it custom to your car and its mods.
2008 M3 Sedan + catless + Tune
2012 CLS63 AMG
2014 M6 Coupe + catless + KW V3
2015 Porsche GT3