think of it as a living space for the adults.. they need somewhere to hang out and mate
by adding some material like this to the culture
you increase the amount of kickback and roaming space within the culture.
I use slightly crumpled wax paper stuck into the culturing medium. It gives both the larva and the adults more living space. I use a piece of wax paper big enough that when crumpled up it just about reaches the lid. Wax paper is cheap in the sense that one roll will last me forever.