Well, several projects use google code. libtcod uses bitbucket afaik.
They're basically a free repository with a web interface. There's either a hg or svn repo behind it that does version control. It especially makes it easy to work with a team, as you need a server to put your repo on, so the other team members can access it. And servers aren't cheap.
In my opinion, they're only useful if you want other people to have access to the source code and possibly help you with it. (Others can't change the code if you haven't invited them.) If you want to be the only one working with the source code, they offer very little a blog/site and a private repo on your HDD doesn't.