The Arms, Legs, Fingers and Hips all have a predefined set of blendable IK targets.
This allows you to quickly have your hands quickly grab and hold your head, hips or knees. Or, you can have your feet parented to another object and smoothly blend to be parented by another object.
Imagine this, your character is walking along, jumps on a unicycle, pedal for while, flies off the unicycle and lands on a pogo stick on a boat. Each of these different parent objects can be animated and adjusted even after you have set up the IK blending.