elastic

I noticed that Pampers Easy Ups has some superior elastic siding/waist. The construction seems to be elastic fibers which take the tension, and an excess of the nonwoven material that bunches up into the folds you see. I wonder whether they will eventually move this over to UnderJams, with their...

I know that having elastic waistbands on both the front and back of a diaper is considered a plus, but I'm not sure what function the elastic actually serves. Can someone shed some light on this subject?