A:Answer I could be wrong but, if this is a 2 channel LOC, then you'd be feeding in 2 audio differential sources. This would make sense if you're getting music from both the Left and Right channels (front or rear, though I'd suggest the front for better quality). To answer your question, the R and L gain apply different levels of gain to their channel respectfully. It's not normal that the audio levels of the Left channel to be wildly different from the Right but, if they are, these knobs would allow you to balance this.
Hope this helps.