It is not different to c-squared, that is true, as c-squared re-steems everything it upvotes. C-cubed do a daily post on the posts it upvotes, so the feed in this case is a different way of viewing that post.
Curie is different. Curie do not re-steem every post that they upvote, or provide a post containing all of the upvotes, so viewing their curation feed is the only real way to see the posts that they are upvoting in one place.