Topics are not hashtags or communities.

Communities can veer off topic.

Hashtags require user input.

Topics are collections of notes that fall under the same umbrella. For example, a topic of Nostr could contain: nostr, fiatjaf, nips, nsec, npub, nostr address, relays, developer update notes etc...

Topics are passive, and do not require user tagging - which is the issue with hashtags.

Communities require "joining" - something people may not be willing to do just to look up a topic.

Topics also help understand what's happening around the world - essentially turning clients into places to get your news. This is one of X's biggest selling points (news).