One reason would be for multiple World Composition based worlds since all the logic for the World Composition is part of UWorld, so you cannot stream in a World Composition based world like you can levels, instead you have to change to a new UWorld. For example, my game has multiple worlds, somewhere in the 20’s at the moment, each of these exist as their own UWorld, then I have multiple interior cells you can travel to from these worlds, they exist as levels and can be streamed into these Worlds.