Boost, to my understanding, is only a thing on Kbin. Ernest coded Kbin to have better connectivity with Mastodon (Boost was originally our analogue for a retweet) then Lemmy does, and that's one of the options he made (the Microblog section being another. The non-Kbin instances (so all of lemmy, and thus most of the people who can read this thread) just don't have a Boost button.
That being said Boost has changed a bit in terms of how it works since it was created, and it's gonna keep being changed (literally they have said they are working on it lol). For me I basically Boost anything I upvote as a force of habit at this point.
Edit: And to those even from Kbin who don't know since I see some of you in the comments - It's currently like a super upvote. Boost something and it'll make the comment rise higher in the thread, like higher upvoted comments on a reddit thread would go up to the top. (Less if it's changed since the time I learned that from some other fediverse schmuck lmao).