What is Bluebeam's "Dimmer" Feature Used For?

There are so many tools and features in Bluebeam Revu, that I guess I shouldn't be surprised when I find something I didn't even know existed, but the "Dimmer" feature sort of had me wondering what it's actually for, so I dug in and took a look.

What I discovered, is a feature that's really useful for quickly finding all the markups that have been placed on a page. Especially when there are markups that are just black lines that are hard to tell apart from the underlying PDF content. Take a look...