Sometimes I want to do a quick edit without having to cd directory, make sure I'm up to date, edit file, add/commit/push just for a md file in the repo
It wasn't a critical update, but one that wouldn't have happened if I had to fork/clone/push/pull-request it.
I totally get why you'd want to do that.
Perhaps they don't like fiddling about on the command line, their IDE is too klunky, file managment is a pain, their version control interface is convulted, or they are not sitting at their computer with their codebase - who knows!
Either way they appear to find it easier that way.
Documentation or fixes to comments in code is an obvious usecase for online edits cross-language.
If you'd really rather do the pull,open,fix,commit,push i would argue that you are too stuck in your own ways.
It's only a useless feature, if you think people who don't think a service by BitBucket will ever be used by people unfamiliar with git and programming.
I for one am very pleased with the opportunity for crowd-sourcing fixes and improvements to open-source websites and projects - especially for people whose interest in them is not grounded in the programming aspect.
As long as you have something like this in place, it doesn't matter how the change gets committed to the repository.
It's fairly common actually, like the PHPs running in shared environements.
Many programming language's websites let you try running some code from the browser, without having to install anything as well, see for instance http://dlang.org/ (amongst many others).
I'm not sure it would be very useful (to me at least, but I mostly write C) but it's definitely not rocket science.