A few more comments:
The powerpush unit (or PPU) is a remote controlled device, unlike the tractor used for pushback which is manned by the ground crew. From this comes the big difference, who is responsible for aircraft movement at this stage? I think the PPU technique may be in advantage because it does require less coordination between pilots and ground crew (as any coordination requirement introduces some risks too).
Then, after seing a picture of such a PPU, it's much smaller than a tractor. I suppose (in fact, rather speculate
) that it may be usable in confined places where there is no room for a tractor.
And, of course, it's about ease of mount/dismount too as Christian has pointed out.
One more thing (though I promised myself NEVER to argue on terminology), the fact of moving the aircraft backwards with its own power is called "powerback". Some virtual pilots on IVAO use to make fun of unsuspecting ATC like "this is an ATR, clear me to powerback not pushback, ha, ha".
In FS almost any aircraft is able to powerback ( except gliders, I presume
) but in real life this technique is very restricted due to collateral risks. A B737 ditched into the Potomac after a failed take-off years ago, under heavy snow, due to engine power loss, and the investigation concluded that one cause were the pilots "helping" pushback by engaging reverse thrust. It was supposed the engines sucked in snow in the process.
So I think another reason for allowing this preferably to tail-engine aircraft is that in this configuration, engines are generally farther up from the ground.
Andrei
