For machine quilting, there is a more detail that shows in the quilting than if you use 100% cotton. It has the characteristics of cotton, but not the shrinkage, so less of the "old-fashioned" quilt look if you don't like that.
I know that people that hand quilt like it better than all cotton as it is easier to needle through.
I like cotton batting the best, as I do like the old fashioned look, but if I want a quilt with a bit more loft, and the quilting to show more, I have used the Hobbs 80/20 and like it.