I’m constantly running into multiple groups that address the same subject. It would be nice if I could combine those so I saw all the posts in one place. For example, all the forums that are discussing UAPs it would be cool to just look at a single thing that combined them all. 
Yea, honestly I think this is where an app can really shine vs using the website (currently).
Another example: Formula1 communities are on lemmy.world AND lemmy.ml. As a user and member of both, I don’t care which one I’m seeing/engaging with, I would rather they be grouped together so I can see the content from both at the same time rather than having to go to each community.
I guess this would be akin to Multireddits (I think it was called?) where you can pick and choose which communities you are subscribed to are grouped together. Meaning you could also do this for communities that don’t cover the same specific content, but a more general category, such as “computers” and have all computer related communities you subscribe to in that group.
Yup, I remember Reddit had something like this.