It’s not nearly the same as following communities or groups, it’s just a collection of posts grouped by tags, as opposed to a space where people discuss or post about a more broad topic. Also Communities and groups typically invite more interaction than simply tagging posts by virtue of being a place people post as opposed to simply being a post tag category.

I should note that there are groups on Mastodon (Not really in Mastodon itself but federated Group actors from other services show up there) though they are less intuitive and thus are usually overlooked by most Mastodon users.