What are selection sets ?
This addon create some collections as selections sets, to store elements by their type. It also make a unique set for Instance Collections.
It will not move objects, it will only link them in those additionnal collections.
Why it's useful ?
With this addon, you can clean easily select elements by their type (Lights, cameras, grease pencil, etc), without wasting your time, because it can be used everytime you want to update those selection sets.