Hello. Could you please tell me a few common cases where I need to use this in animation instead usual moving and keyframing?
In character rigs, this is commonly used for IK to FK switch, muscle/deform corrections, and facial controls. It is also useful for parent switching. Like a piece of equipment that a character uses a lot and may have it in the right or left hand. Yes, you can do it manually, but it it's used a lot by a character then animators love this simple addition to the rig.