I often have 100 tabs open so that I can find things I was looking at again. I regularly browse my browser history to try to find things I saw previously. I often remember seeing something in a chat session somewhere, but can't remember which person/channel/room it was...

Being able to search everything I've seen on my screen would address all of these. I think it would fundamentally change how we interface with computers, if we could do it reliably. Silos between applications can start to break down when you have sufficient intelligence about what's on the screen too, and that's a huge opportunity.