Level Streaming is an Unreal Engine technology to asynchronously load/unload content inside a single UWorld instance:
The Level Streaming feature makes it possible to load and unload map files into memory as well as toggle their visibility during play. This makes it possible to have worlds broken up into smaller chunks so that only the relevant parts of the world are taking up resources and being rendered at any point.
With MultiWorld instead, you can have multiple independent UWorld instances active at the same time.
Simply toggling the visibility of a level at run-time can impose a number of issues to the gameplay (e.g. to the physics), to the graphics output (e.g. with static lighting or different real-time global illumination setups) and other systems. While it can be used for simple scenarios, it's something that must be carefully planned, tested and that can impose severe technical and artistic limitations.
For example in a multiplayer game, if you need to show the “inventory scene” to the player, using Level Streaming to hide the game-play world would break the game. This works instead with MultiWorld.
Note that, being the Level Streaming basically an asynchronous loading feature, it can be used with MultiWorld to nicely handle the content inside a UWorld instance. This is true also for World Partition.