Controller file vs controller function

[eluser]chiste[/eluser]
I have been fallowing the "Codeigniter from scratch" screencasts at net.tutsplus.com and I have the fallowing doubt about when or how you choose between creating a new controller function using the site controller file and creating a new controller file with new functions. it seams that you can fit all your controllers functions in a single file.

I'll appreciate the any explanation.

Thanks


Carlos