You have a keyboarder who plays the Korg Kronos and he wants to play the Kronos German Grand which has the keys mapped from low (left) to high (right). If you e.g. just send the left channel as a "mono" feed, FOH will not hear the upper keys.
Most decent keyboards switch to mixed-to-mono mode when only one cable is plugged in- in that case both channels of a stereo patch will be mixed to mono and output together, so nothing goes missing. It would be a huge surprise for me if the Korg Kronos didn't have that feature - I'd consider that a must-have feature on any professional-level keyboard...
Also, a keyboarder performing live on a regular basis should be prepared to provide a mono signal if needed (if your keyboard doesn't have that feature, there's simple DI-type boxes that can to that) - that's just plain professionalism... Unless, of course, if you're so exalted that you can dictate the terms and just _demand_ that you can feed in stereo...