Not sure the question has been answered: Jelly Roll is 2.5"xWOF (42-44") . Charms are 5" square and Layer Cakes are 10" square. You generally get one (or two) pieces from an entire fabric collection depending on how they are bundled. If you want to have the selection of the entire collection, then these precuts are a good choice because buying yardage of each print in the collection would be more expensive. Yes, you pay more if you review yardage price but it may be worth it in the selection.
I have bought Jelly Rolls for the Jelly Roll Race and for the Summer Fun quilt from Jenny Doan's pattern. I have used charms to make Apple Core cuts (little waste) and I also use them for DP9 kid quilts.
Precuts are a great selection when you don't want to (or don't have time to) cut your own yardage.