On Lemmy you don’t need them. On person-based social networks such as Mastodon or in this case Friendica, they help with visibility. Basically you follow hashtags and you can find lots of other people based on the content they make (that you’re interested in - i.e.: you follow #trains and you get all sorts of posts regarding trains or railway in general). This also means that you can post something with a hashtag and lots of other people who are interested in that topic can follow you back.

I do see why it might suck though. I rarely used them when posting to a Lemmy community or any sort of group on the Fediverse (mostly only when I wanted some extra engagement).