It's pretty useless feature I would say and only related to how your feed behave. Probably we only could appreciate its full function in a very distant future update...LoL
Just add friends to anyone you want to follow. It is a simple and more straightforward way.
BTW this is not true AFAIK

Depressed Willi:
but you cant private message (DM) the person
You can DM anyone in MDL (friend or no friend) unless you're blocked by the person.