I wasn't at first either. It seems unnecessary at a glance. It does greatly simplify accepting contributions from users without commit rights. For outside committers, they can commit as much as they need to without affecting the baseline. Which makes things much easier for larger tasks. Pull requests and code reviews are much simpler than reviewing a patch file. In general, merging branches is less painful. Just my 2 cents. History is maintained with the conversion, in case that's a concern.
So as someone that wants to contribute to this project, I'm asking the question because it would make my life easier.