If it is not possible to login as root directly (which on a multi-user system, it might only be possible to do if you are directly at the computer), then the wheel group might be one of the pieces (which should not be used alone; the wheel users still should avoid revealing the root password anyways) to protect against accidental telling the password, I suppose, since then other users cannot use the password without physical access to the computer. If they deliberately want to do so, then they could presumably just as well make the other users to also be the wheel group (or modify the software on the computer so that it does not require wheel, etc), if you have root access.