It definitely didn't use to track merges. That got added after most people jumped ship to git. Before that, you had to specify the common ancestor commit of the things you were merging, which you usually didnt know and had to dig around in slow logs to find out.
It has tracked merges longer than github has existed, just to keep things in perspective. But, again, it's not required just to keep stable branches around since commit numbering is linear in svn. It's very convenient for anything less trivial than that though.
