I am eyeing a Denman brush, having heard about them here. I hear they can give "curl definition". What exactly does that mean?

I have 2B layered hair that can be limp and unpredictable. Older daughter has 3A that is curling pretty nicely lately in big ringlets. Younger daughter has 3B that is prone to frizz. We normally use a wide tooth comb or a pick.

What benefits might we get from a Denman?