The butter not only adds richness to the burger and prevents it from drying out, but it also butters the bun for you. We're not going to downplay it, these burgers are decadent. But, if you're going to eat a burger, you may as well do it right. And, this recipe does it so right.
The word persistence means the quality of being determined to do or achieve something. It's tenacity. It means firmness of purpose. It is a continuing, repeating or lasting of something. Use the link and read a few similar definitions.
Mast & Kawin present a definition of persistence of vision in chapter two, under the heading of Persistence of Vision. Their discussion indicates that in the 21st century, persistence of vision has once again picked up steam. Mast & Kawin (2000) go to great details to explain the phenomenon of persistence of vision.