But why would he use a knot that is less strong than others (like just using a double fishermans as the bend in the first place)? With modern ropes, sure, having a square knot eat up half of the ropes original strength is adequate for rapelling. But why would one go to the trouble of tying a square knot, and then two double fishermans knots as backups when they could have just tied one double fishermans in the first place...

Because rapping on 2 ropes tied together with a double fisherman's can be quite hard to untie, especially if the rope got wet or you're rapping with heavy loads (i.e. haulbags while retreating from a big wall). The square knot keeps the fisherman's from cinching down tight on itself.

BillyM is correct about the overhand w/ long tails. It's also been known as the EDK (Euro Death Knot) after its importation from Europe. The body of the EDK stays to one side of the loaded lines and thus has less tendency to hang up over edges when pulled. It's very simple to tie when tired & cold.

The IntardNet used to rage with arguments against the EDK but most accept it now.