Why it's good for you:
It puts delicious pressure on your G-spot. There's also the sensation of being in complete command of events and being able to use your vagina at any angle you desire.
Why it's good for him:
The delight of this position is that he has very little to do, except perhaps bounce up and down off the bedsprings. It also stimulates his penis and gives him an arousing view.
How to vary it:
You can lean forward for breast and mouth kisses, or backwards to show yourself to him. From a kneeling position, manoeuvre one leg so that it's under his thigh, then bring that leg over and rest it on his shoulder. Now, still sitting on him, you can lean back so that his penis applies maximum pressure on the front of your vagina – and, hopefully, on your G-spot. This is known as the "X position", and often helps prolong the pleasure. It's often difficult for the woman to balance in this position and she may need to grasp the man's hand to steady herself.
Watch out, you might catch a speed wobble.