I could toss 2 ideas out there if your refering to a straight knee.
One being obvouse, you get more power if your lower leg is tigher to your upper leg(you can feel the diffrence from trying both ways)
Second maybe a protective mesure? if your using the front of your knee, and your dont pull your lower leg tight with your upper leg your ACL and all those injury prone ligaments look to be on the front of your "weapon" more.
Again these are just speculations, when i started MT it just felt better to do it the proper way so i never had to ask why.