It's great for two things. First, for making smaller quantity recipes. A crock should be half to two-thirds full and when you put in a smaller volume, it doesn't cook properly. You can put a smaller dish into the crock and cook that way, which I have done with success for baking sweet breads. But since my big slow cooker is oval, large round dishes won't fit.
I've made some great soups in crock jr already. The broccoli soup, which was perfect on a chilly day and mushroom soup, which tasted nothing like the stuff in the can.
I also like it when I want to make a side to go with whatever I'm making in the big crock. Last week I made chicken drumsticks in the big crock and soup in crock jr. It was a breeze!