One of the benefits in addition to being able to read fossil source code and being able change/style the web interface however I want is that I don't have to worry about staying up to date on Terms of Service changes I really have no say in.
Edit: failed to mention fossil can import/export to/from a git repository[3]
[1] 2-clause BSD license: https://www.fossil-scm.org/xfer/artifact/f99187d1905883d3
[2] cgi script: http://fossil-scm.org/xfer/doc/trunk/www/server.wiki#cgi
[3] import/export git: http://fossil-scm.org/xfer/doc/trunk/www/inout.wiki