It would be pretty similar but more flexible, because the algorithm for choosing what to show can be more complex.
As an example, suppose a smaller community usually has usually posts which average 1-5 likes. If there was now a post that recieves let's say 30 likes, that would be a very good post in context of the community but when looking at at the top of all it would possibly be overshadowed by bigger communities with higher average likes.
This custom aggregation of posts could allow to highlight such 'high quality' posts of smaller communities.
This is just an example, the actual selection could be more intricate.
All is nice but I haven't figured out a sorting setting I like there yet.
Sorting by new shows a lot of lower quality posts.
Sorting by most votes is dominated by bigger communities.
Using aggregator bots would allow for custom sorting without forcing it on everyone.