Unless there is a gross imbalance, it is hard to know what one is missing without such a control until one gets to experiment with such controls. The best way to hear what happens when you change balance is with remote-controlled balance adjustment (instantaneous comparison). Even extraordinarily small changes can be heard, and better balance does improve the sound. I use to have a Mark Levinson No. 32 preamp in my system. This unit allowed for .1 db increments of change in balance and/or volume. I could not reliably hear even a 1.0 db change in volume with most music, but, a change in balance was another matter; I could easily hear a 0.2 db change in balance. When center images where perfectly placed, the sound was better to me. With the Mark Levinson, I could permanently program different channel balance for different sources, which helped with my phono setup because my cartridge had an imbalance of a little more than 1.0 db as compared to other sources.
I insist on some form of balance control, which is second only to remote control of volume level as a must-have feature on a linestage or preamp.