Those two straps are for situations when you don't want to take the back window out but you do want to roll it up. They straps snap to the back bar on the tops frame, then you can unzip the window to the point where it is hanging just from the top portion and roll it up, then loop the straps around the ends to hold the rolled window in place. This is easier than removing the entire window. Becareful while rolling it up because the dirt on the plastic can leave scratch marks.
Does anyone know where to get these straps? I had two but one went missing one day in a parking lot.