If you want your posts ranked better not just on Nostr itself but in search engines

Instagram started off hashtags as a way to filter content on the site when searching

Later twitter took over the idea but then search engines saw how popular they became

So search engines then used them to help filter messages from social sites in search engines

So in turns of hashtags the term is for social networks/media and search terms are for search engines so now in a way they are the same thing almost you just use # on social and without on search engines

Sp it's up to you but posts I want found better I use hashtags and general posts where is just an update or just chatting I don't bother