Because when you develop applications you have to compile your code frequently,
you are then doing cleanup before compiling again and what you do is...
compiling unmodified sources again and again and again, loosing a precious
time doing the same work for nothing.
So, when you achieve some useful and stable code, you know that it won't change, so why would you compile it again? There is another solution for that!
You can create a library
<mylib>.a and this is great, but, you have to deal with all the architectures (arm, i386, etc.), and this is not really convenient, even if it exists methods and script to merge different archs into one file.
So there is another elegant way to do an equivalent job.
How to make it work
First of all, you should install the iOS Universal Framework templates (Real Framework and Fake Framework).
This is as simple as is, you can now create a simple real framework for iOS and compile faster.