Learning UE5, levels & world partition

I’m trying to learn more about UE5 to optimize my scenes and use them for film. I’m going through some 4.27 tutorials that work for UE5, and have a few questions.

Why use levels in layers? Currently I have a save with my nDisplay settings. When I start a new environment, I copy everything from that save, and paste it into my new level. What does adding it as a layer do?

Same question for level snapshots. The changes you make to materials doesn’t revert, so why not just make saves with version numbers?

And world partition; I haven’t investigated this much. What are the benefits/reasons to use it?

I’m trying to learn how to optimize things and make environments run quickly for filming, and could use any and all advice about what I should be looking into.