That's not a protocol thing, though. Clients can display AP or Mastodon posts as feeds, much as e.g. Phanpy do.
Feeds are a fundamental part of AT and depend on relays. There's no equivalent in Mastodon. It has relays, but they're more niche and restrictive and there's no standard way to filter, select, order, etc incoming posts.
The firehose used to be easy to get to on Twitter until they realized people would pay for it, and then all the fun mashups went away. The firehose and feeds that filter it are a fundamental aspect on AT.
It makes more sense once you've tried it. What we're seeing with Bluesky feeds is what Twitter could have been if they hadn't tried to turn the firehose into a profit center. It's hard to explain because it sounds like tags or saved searches if you're trying to relate it to the more common model of social media.
But I agree that there are limitations with using an instance centric approach and the benefits may not appeal to many people.
That's what I meant with:
>> "It's hard to explain because it sounds like tags or saved searches if you're trying to relate it to the more common model of social media."
For example: a feed that's something like a "For You" timeline without the commercial incentives to make it bad (it's empty without an account because it has nothing to work with): https://bsky.app/profile/did:plc:tenurhgjptubkk5zf5qhi3og/fe...
It's not just blunt filtering. Feeds can (and do) apply machine learning, take input from your interactions, and generally do all the things a first-party timeline can do.
The default feed only shows people you follow, and if you 'bookmark' (can't remember the correct terminology) another server you get a feed that shows only that servers their timeline.
One of them being the feeds: it really does work well. On Mastodon I can follow people/hashtags or even multiple servers (e.g. with Fedilab), but they just don't get organized in feeds. And it turns out that I am more interested in feeds than in servers.