It seems like nothing, but it's very handy to be able to hit F2 and see if your code works, then immediately go back to it.
http://ed.cranford.googlepages.com/vimrc if you're really interested in seeing my vimrc. I'm not fond of plugins.
:nnoremap <C-P> :w !python<CR>
:nnoremap <C-P> :w !php<CR>