It is the L side controller dedicated to the portable mode. By installing it on the Nintendo Switch body, you can operate the cross button for easy enter commands. The following functions in Joy-Con (L) cannot be used; Player lamp, synchro button, SL button, SR button, HD vibration. This product is exclusive to the portable mode and doesn't perform wireless communication in TV mode and Table-mode.
Important Notice:
The issue for the Nintendo Switch console's battery draining while the accessory is plugged has been fixed by its manufacturer, Hori. If you find this happening to your consoles, make sure that your firmware is updated to refrain this from happening.