Well Physt, for the reason that it solves the problem Joe just illustrated. If you create a separate pivot for the collisions, you can have your collisions at the height you want them ( knee, waist, head, whatever ) and still use the centre pivot for rotation and world positioning.
The other thing to bear in mind is that you're probably going to come across a situation where one set of collisions per character is not enough. If you have a pivot down low and another up at head height, you can use the collisions on both to have your character jump over things, duck under things, etc automatically.
EG:
collision low and high = wall - stop
collision low but not high = step or low obstacle - climb over
collision high but not low = low ceiling or passage - duck
The other thing to bear in mind is that you're probably going to come across a situation where one set of collisions per character is not enough. If you have a pivot down low and another up at head height, you can use the collisions on both to have your character jump over things, duck under things, etc automatically.
EG:
collision low and high = wall - stop
collision low but not high = step or low obstacle - climb over
collision high but not low = low ceiling or passage - duck