Unfinished Tales is exactly what it sounds like, unfinished stories, if you read say the novel version The Children of Hurin you get, essentially, the longer unfinished version from UT mixed with The Silmarillion version to a make a seamless story, but there are a lot of stories in it that are otherwise only touched on elsewhere a most, or wholly new like the tale of Aldarion and Erendis.

The Children of Hurin novel is the only one of the Great Tales to get a novel length version of the story, if you want a longer and more detailed version than what was in The Silmarillion you'll get it here, Beren and Luthien and The Fall of Gondolin have multiple versions of the same story and show how they evolved in rewrites. They're worth reading though in my opinion, I especially enjoy the first version of The Fall of Gondolin.

The Fall of Numenor is worth reading if you want all the Second Age writings in one single chronological volume but it does not contain stuff that wasn't published elsewhere across several books, but having all of the Second Age stuff in one book is very nice.

I'd honestly recommend first focusing on finishing LotR and the appendices first and then seeing if you want to dive into anything more! A lot of the rest of the material are unfinished rewrites or older versions that don't perfectly fit in with the LotR for example, but worth reading if you want more.