Anyone here know why the charging handle on the M16 was placed in the rear?
I'm guessing it is the same reason that I need to fill my vehicle's gas tank from the driver's side, rather than the passenger's side.

Is this an advantage/disadvantage compared to a side charging handle?
I think it could be viewed as an advantage. If you are correcting a malfunction by using your non-firing hand to push the charging handle back, while keeping your firing hand on the pistol grip and the weapon oriented in the direction of the threat, you can correct a malfunction without moving the muzzle all that much. A side charging handle, it seems to me, would result in the muzzle moving a lot more, thus taking an extra second for you to get your sights back onto your target.