Judging by the response linked-to above, it does provide some usability advantages when the installer is used as intended; specifically, it makes it harder to erroneously attempt to treat the installer as an ordinary archive, which, judging by the description, could lead to broken installations with the cause potentially unclear to the user.
For example, to quote from that post:
There were situations, when users would download just a single part of the installer, or try to unrar it manually (because apparently some browsers detect our new archives as rar files), or even try to open the .bin files with the VLC Video Player.