My unit tests did catch it, but if you've ever (seriously) coded in a statically typed language, you really like to have those errors show almost as you type.

To give you an example on how i do small refactoring in objective-c :

I change the function (or class) signature i know i need to change. I compile, then xcode shows me every single line of code i need to change.

Most of time, it shows me places i didn't remember also used that function (i'm talking "utils"-like functions).