I built this jig when I was making drawers for my tool cabinet and needed a way to secure them to my workbench while handplaning them. It's just a piece of 3/4" plywood with slots that are spaced apart to match the width and length of each of the two sizes of drawers. The width of the slots equals the thickness of the drawer boards. That way, the drawers don't shift at all when you plane them. This jig also enables you to plane from both directions so you don't tear out the corners.
If you're making drawers that have a bottom that slides in after the sides have been glued together, then it's even easier to plane the sides. Because without the bottom, you can slide the drawer further in from the outer edge of the jig, providing even more support. (see last photo)
By the way, these drawers have walnut sides and cherry fronts and backs. That's one of the backs in the last photo. Notice how much lighter in color that cherry is compared to the fronts, since the backs never see sunlight. The fronts have naturally darkened that much (I only used blonde shellac) over two years.