So your post about keyboard layouts or databases will more likely be discovered/boosted (even if you have no followers on other instances), because the local timeline and hashtags are more quiet than when you're on a general instance (where your posts drown in the noise). Or compared to, say, sauropods.win, were you more likely reach a geologist than a database admin.
Generally I like to be on two or three instances, so my art doodles go to mastodon.art rather than a tech instance (flooded with AI art). I like to check the local timeline or discovery page of other topic-instances via web occasionally, maybe follow someone there to peek into another bubble. The big instances like mastodon.social are just too much, flooded with random stuff I don't care about.
I could have five different accounts on five different instances on topics I'm interested in, but then which one do I log into if I just want to read my overall feed? I guess I'd have one primary account, and then I'd log into my secondary account on the paperclips instance when I wanted to toot about paperclips, but this is the point where the UX seems awfully complicated.
Talking on a social network on the Internet is not the same as talking in real life. Unlike Twitter, Mastodon recognizes this.
Maybe all you want to talk about on the Internet is Emacs. Maybe you want to keep your "Emacs nerd" persona totally separate from your "classical music afficionado" persona. Maybe you are part of a group of Emacs committers who want a casual place to hang out.
Outside of the small hassle of adding people, there are no real downsides to joining a random instance.
[1] Outside of super controversial or illegal topics.
It's all about choice.