The red line traces the mouse movements. The red dots indicate clicks. I see what devices and browsers my visitors have used, and how long they've stayed.
Following what my users saw I can test different screen sizes and devices with eyes of my users, without setting the environments up myself. Doing this early on (and fixing), I could prune out problems through testing in production, annoying a limited number of users but coping with my limited ability to cover different combinations.
For now, I'm just blown away with this. And needed to share. I reserve my right to change my mind as always, but for now, I'm just excited.