RawGit serves raw files directly from GitHub with proper Content-Type header

To download an image asset. To download build that is checked in to github as part of the build process.

Doing it through the github web interface takes a lot of clicks.

In making demos on jsfiddle

Also useful when inserting scripts in a dev console.

probably because they don't want to become a free CDN for people

That is answered in the FAQ.

>They do this because serving raw files from a git repo is relatively inefficient, so they want to discourage people from using their GitHub repos for static file hosting.

To prevent using them as a file host. If you can just dump files (of whatever size, it doesn't really matter) onto github and serve them natively across the web then they become a file share or a CDN. They'd have to seriously lock down the rules on publicly available free repos if that happened (and they'd have a copyright nightmare on their hand as well). Whereas if they force git into the loop on the client end then it keeps them in that repo hosting butter zone.