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).