I’ve never heard of circuits that were not supposed to be ungrounded. I had always thought all circuits needed to be grounded in one way or another. That shows you how much I know about electronics.
Anyway, the ground detector is a very useful device indeed. You should at least understand the concept. It’s amazing the number of people who will buy a surge protector and ignore the grounded warning light which ensures that the protector is properly grounded.
If that light goes bad your protector will fail, and it won’t matter how many “joules” of protection were advertised on the product packaging. It pays to know at least a little about these things if you are going to protect valuable equipment like personal computers.