OK, as an only semi-literate neophyte, let me try to explain it as I understand it.

Picture your rear wheel from the rear. On the left (non-drive) side, the flange with the spoke hole is out close to the end of the axle where it fits in the dropout. On the right (drive) side the flange is inboard of the freehub body or freewheel. That is, not out on the end of the axle near the dropout like on the non-drive side but closer to the center of the axle. So unlike the front wheel, the rim is not centered between the flanges but it must be centered between the dropouts. The spokes must therefore have more angle out toward the non-drive side flange and come closer to straight up and down on the drive side in order to get the rim centered. Now, you probably know all that already since you have built wheels so forgive me for stating the obvious. From what I have read on wheel building a dishing tool should probably be called a centering tool. Its purpose is to make sure the rim is centered regardless of the dish (offset) of the spokes. Any accurate way of centering will work. The dishing tool just makes it a little quicker to accomplish and possibly more precise as someone (Pat?) noted.
Steve and Pat, correct me if I am wrong. I only reply on these things to try to give a complete beginner's level explanation as I understand it. Sometimes I have to read about something in several different sources or have it explained by several different people before I feel like I have a fairly accurate understanding. So I am just trying to help flesh out the picture for Claude.
Regards,
Raymond