OK that link totally didn't do what I thought it would. Anyways, what I meant to say was: see Tycho's post below - sometimes you're at a PC which doesn't have Python installed.

So it doesn't have to be a replacement for your desktop, although it can be. It can also just be a complement - if you use dropbox or github, then you can know that, no matter where you are, you can always get to your code and work on it... internet cafe in Thailand, friend's PC, locked-down-corporate-desktop...