Why do some recipes call for sugar in a brine?
The sugar is simply used for flavoring; the fact that it helps brine to a lesser extent is just an added bonus. The sugar also aids in browning via the Maillard reaction, though this can also result in burning in a high heat application.

I suggest brining two boneless skinless chicken breasts -- one in a salt-only brine and the other with the salt &amp; sugar brine suggested by Cook's Illustrated. You should notice a difference in both flavor and appearance.