A lot of people do. Especially on GitHub, where you can just browse a random repository, find a file you want to change, hit the edit button, and edit it right there in the browser (it does the forking for you behind the scenes). For people unfamiliar with git, that's huge.

It's also a great boon when you don't want to clone the repo locally! For example, when I'm on a slow, metered connection, I have no desire to spend 10+ minutes (and half of my data cap) for a repo to clone, just so I can fix a typo. With the web editor, I can accomplish the same thing with very little network traffic, in about 1 minute.

While normally I prefer the comfort of my Emacs, there are situations where a workflow that happens entirely in the browser is simply more practical.