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.
In computer science, persistence refers to the characteristic of state that outlives the process that created it. This is achieved in practice by storing the state as data in computer data storage.