Face Down is a position that provides a lot of skin-to-skin contact, especially if the giver lays on top of the receiver. As a more dominant-feeling position, it can also feed into the receiver's fantasy of being 'forced' to accept the pegging. A simple moving up or down will change the angle of the toy to target to prostate. The giver is also perfectly positioned to whisper naughty things in the receiver's ear.
Face down can be more comfortable for both parties with a couple of pillows under the receiver's hips, or an awesome Liberator Ramp sex cushion. This is their black label, which has restraint points.
The giver can put their legs between the receiver's, or outside of them. A simple thrust of the hips is sufficient to fuck the receiver in this position.
Being able to lay down on the receiver makes it easier for the giver, and qualifies it as a good beginner's position.