Writing unit tests is important. Without unit tests, how do you know that your software still works when someone makes a change? Sure, you can deploy a small army of manual testers to double-check that it still “works.” But maybe the change you made has subtle effects in disparate parts of your code–are your testers going to cover everything?
The best way to have confidence that your code is functioning properly is to build and maintain a suite of unit tests. There are entire books written about this topic (for iOS developers I like Test-Drive iOS Development by Graham Lee), but the most recent issue of objc.io covers just this very topic and is a great place to start if you’ve never written tests in Objective C.