With divider you can separate tub for 2 compartments for smaller snakes.... if you plan buying small balls in the future (hatchings, yearlings) set of two can be useful.. if You decide buy rack for them check they hide selection they are very nice as well... I'll use medium hide up to 1000g and large for my bigger male...
Hope that helps
Edit: this is a link to tread about RBI dividers with exact rack I use...
http://ball-pythons.net/forums/showt...t=tub+dividers