I am an iOS developer.

I have read about unit testing and how it is used to test specific pieces of your code.

A very quick example has to do with processing JSON data onto a database.

The unit test reads a file from the project bundle and executes the method that is in charge of processing JSON data.

But I dont get how this is different from actually running the app and testing with the server.

So my question might be a bit general, but I honestly dont understand the proper use of unit testing, or even how it is useful; I hope the experienced programmers that surf around StackOverflow can help me.

Any help is very much appreciated!