This is so you can show/hide the rig or character really easily.
It just makes it easier to work.
Although in 2.8 this is becoming 'collections'. So it's similar, but you want to organise your collections so you can only show the rig(s) you want to work on.
its what i suspected , it was driving me nuts, i kept reading about layers and not finding the same options and thinking this sounds just like collections
