Probably nothing; if anything, it's a subset of git.
> or what its purpose is?
To be a saner subset of git with a more usable frontend.
That being said it's not a drop-in replacement and no one uses it, so it's not as if you can enjoy how much better it is very easily.
http://wiki.netbsd.org/github/
https://src.fossil.netbsd.org/
Sadly, somehow the transition to the next VCS has been set to HG, which really makes no sense to me at all; Mercurial is not even BSD licensed, whereas Fossil is; plus, HG is just dead (even Bitbucket discontinued it); the decision to go HG in 2020+ really makes no sense, and I didn't even see anyone argument it; hopefully, it's reversed before it goes through.
Unfortunately as far as I'm concerned it's over. Git has won. It's just way too popular now, other DCVSs can't compete.
I'm getting tied of git and disappointed no one wants to get off of it instead more are using it.
https://www.red-bean.com/kfogel/beautiful-code/bc-chapter-02...