As said before in this thread,
My primary use-case for this would be most definitely player x-ray.
I have a game in development that has a character with a special ability to see enemies through walls. Within the current roblox toolset, this is actually quite hard to accomplish without completely wrecking performance.
For RTS-like games this also has a use-case, where if you select an item, this silhouette would surround the object, notifying you of the selection.
There are a lot of uses but these are the most important for me