Also, for pair programming it gives the pair a common goal to aim towards. The people know the inputs, and outputs up front. So one person doesn't just stare

Also, for pair programming it gives the pair a common goal to aim towards. The people know the inputs, and outputs up front. So one person doesn't just stare into space whilst the other person codes the function, and then says "this is what I wanted it to do". It also helps reduce code, since you only write code to pass tests. You do not write code for some general theoretical use case that is never required.