Communities simply help people stay on topic, and provide a way for moderating members if need be.
Circles allow posters to choose who they want to share posts with. Admittedly, this is a feature I haven't used much since G+ allowed asynchronous following (i.e., you can follow me and I don't have to follow you back) and Collections.
Collections by far for individual posters is the most useful, because it allows others to choose what they want to follow from you. If you like someone's photographs but don't want to know about their baking, you follow their Photographs but not their Baking collection.
These things are honestly what a lot of people have found lacking in other platforms. MeWe has held an AMA or two and most of these features are in their pipeline in one form or another. The concern is currently there is no public posting. Their UI is slick, but it's just not grabbing me, likely because of the siloing from not having public posts. Pluspora (and Diaspora et al) lack things like being able to edit posts and comments.
There's an articleI can't find the link to right now that really nailed why Google+ was so good for the users who were really dedicated to it... And the communities/collections/circles were part of it.