If you're frequently on-the-go, without your own laptop?

If you're on a restricted machine where you can't install anything, or have other install restrictions (hello, iPad! ;-))?

If you're on a shared computer where it wouldn't be nice to overwrite someone else's config files?

If you're on a mobile device?

If it allowed you do stuff that would be difficult or impossible to do with a local text editor? Like editing and testing web code live, without even perhaps a "save" command?

[Note: I'm not the OP and I don't actually want a web-based text editor for myself, but these are reasons that came to me in the first 2 minutes of thinking about it.]