Natural Sound makes a good point - the biggest reason for a balance control is to allow correction for an ordinary volume control at the lower settings. That's one reason it is usually not needed with a stepped control. There are other reasons, but they are much less frequently needed.
(1) sometimes the exigencies of architecture and communal living require speaker placement that is unsymmetric enough to need compensation;
(2) occasionally, you may have tubes with unmatched gain - usually not significant, but not always;
(3) sometimes you might want to listen to each speaker separately, to identify problems;
(4) you may have unbalanced hearing loss;
(5) the recording itself (or a phono cartridge) could be unbalanced
All but (2) are unrelated to the amp itself, and most people, most of the time, don't need or use the balance control. When PB developed the MourQuiet/BeeQuiet coarse/find level control, I started adding them to preamp designs as an inexpensive way to do something functional with the second hole in the chassis. Off topic but possibly interesting, that's the same way the shunt-regulated driver in power amps came about - looking for something useful to do with the second triode in a dual triode driver. :^)
