I can't speak for everyone, but I appreciate the mirrors because I enjoy reading and searching through codebases on Github, so having the repository mirrored there is useful.
If you look below the repository title on GitHub, it says "mirrored from..". This means GitHub handles the synchronization (they set this up on request for some projects).
For me the advantage is being able to use GitHub's nice UI to browse the source code.
Github has some nice features that chromium.googlesource.com lacks, like being able to list all of the commits that have changed files in a specific sub directory.
Synchronising a mirror automatically is pretty trivial so I imagine they're usually pretty up to date.
> Github has some nice features that chromium.googlesource.com lacks, like being able to list all of the commits that have changed files in a specific sub directory.
fwiw you can do this on chromium.googlesource.com as well. look for link called `[path history]` when navigating the tree.
Aren't mirrors going to be slightly out of date (unless continually synchronised). And you wouldn't commit to the mirror, right?