To develop that big flexor/pronator belly, you need to use wrist flexion. Really heavy wrist curls bother a lot of my lifters' wrists, so we usually go for sets of 8-plus and seek a pump rather than maximal strength.
Wrist rollers are better suited for heavy wrist flexion, and this version is especially challenging since the resistance increases as you roll.
Admit it. A lot of times you plan your workouts around your major muscle groups and just "throw in" a few sets at the end for your ancillary muscles – calves, abs, and forearms.