I am not 100% sure, but I think all that is added is new areas/quests and not new mechanics or anything that changes the game. Just extra stuff in the game world, basically. But there is already a ton of things to explore and experiences to have for just that $15. Money well spent.
Let me double check.
Far Harbor - sounds like a new area on the map, a huge extra land mass to explore. Neat! But no changes to the game, just more stuff. Good reviews though.
Nuka World - sounds like another area to visit, not as big as Far Harbor, and getting mixed reviews. Hm.
Vault-Tec Workstop - Wow, very negative reviews. People must think it’s a huge waste of money/time? More stuff to build in your settlements, I assume. Settlement magagement is mostly why I can’t get back into the game again.
I may have to look into the mod @Lantz mentioned while I was typing this up, maybe.
Contraptions - Also negative reviews, also more building stuff. Is this the Sims!? :)
Atomatron - This seems to change the game a bit by adding an event where robots are more frequently encountered and drop crafting bits to upgrade power armor and build your own robots. Seems neat, getting good reviews.
Nothing (maybe Far Harbor) really does much for me, personally, but YMMV.