Kylo I'm sure there are many use cases. For me the most important is help desk / troubleshooting. You don't want to need to ask a user password to inspect what they can see when they have a problem. It's a feature many websites have for customer support. Depending on the context, you might want to add a PIN feature to prevent abuse, this feature is not available at this time.
It's also very useful in staging environments for switching test users.
The number of downloads clearly shows there is interest.
As for spying on users, an admin can always do it anyway. With this extension they could extend that power to users who don't control the server hardware.
Of course if you don't like the concept, you can simply not install it.