Small topics instances (like emacs.ch) are cozy to join. They are a collaborative filter to focus exploration on a part of the fediverse. The local timeline will be not just about emacs, but has more likely programming posts than, say, clam biology (hashtag clamfacts, if you're interested).

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.