If you have both branches and a way to close them, then you have a much simpler way to get a feel for the history of the code. You know at a quick glance what sort of major work has gone on and what's still ongoing.

If you don't have that you'll end up inventing more complex ad-hoc schemes to get it (merging a branch, deleting it and tagging the merge commit is an example of one such scheme, and it doesn't do the same thing).

And the ability to mark branches as closed in Mercurial is, to me, an indication that the developers put some thought into the different use cases for branching and wanted to cover more of them than git's standard workflow does ;)