There's less of a performance hit at runtime than using a scripting language like Ruby/Python/Lua/Perl/PHP, since the generated code uses a fair amount of compiled C.

You can use Smalltalk-style messaging that C++ and C alone wouldn't give you. So you can query objects for methods, variables, and other things that they may or may not have without causing your entire app to crash.

You wouldn't have to wait at runtime for a VM to startup, since Objective-C has no VM. :)

Just a few things.

One use-case I've encountered before is to take your models from your iOS/Mac application and directly reuse them on the web. In my circumstance, it was compiled and run on Linux, though this seems to use Macs which could be useful if you have a Mac-only API that you use.

For native iOS it's required.

*edit - my bad, it looks like that's not the primary use.