Fossil came out in a time when your options were basically CVS, SVN, or something commercial like Perforce. Git is an innovation that came out around 2005 (A year before Fossil), and it took a few years to get some traction, but holy moly are either of these better than what came before. It's just way easier to mess things up in the old school systems.

I think it's pretty interesting in that it's a hugely replete web application written in C that does all sorts of stuff... bug tracking, wiki, etc.