Esther: Collections are supposed to be for placing all the bits you need for a scene in one place and can be brought in from main and or external runtimes so you can access them from one place instead of navigating all your external runtimes if you have them.
Now you could in theory have all your V3 stuff for instance in a "collection", your clothing, hair, props etc. all for V3 in one place, yea I know you prolly do that anyway with an external runtime but it "might" work better as a collection! Maybe!
Haven't actually tried this yet because I too am waiting until I know my P7 is stable enough to take P6 off!

Injustice will be avenged.
Cofiwch Dryweryn.