Because you can directly review and discuss the changes inline without the need to leave your mua. There is also no need to setup a public git repository and check all your changes in your repository before pulling.
git format-patch, git send-email as well as git am work also really well and simplify the patch creation/integration. This is basically the send a pull request from github but without the need to depend on github.
I've worked in a couple of companies that did this. We had the following flow and it worked great:
Task assigned to developer via email, developer takes current release tar from ftp and untars, does work, creates patch, forwards patch to colleague to review, forwards to release manager who integrates all incoming patches, drops into a new tar, releases to ftp.
Some of this was automated with a few hundred lines of perl. The rest was on a whiteboard.
git format-patch, git send-email as well as git am work also really well and simplify the patch creation/integration. This is basically the send a pull request from github but without the need to depend on github.