The only case when I used sublevels within the persistent one was Level Streaming. I had 3 maps: A, B, C. When the game started, player was spawned in A but B and C were loaded in memory to speed up opening new map. So basically you have one big level which contains several maps (or several pieces of one big map) and you can tell Unreal when it should start loading certain map.
This is well described in docs: