What is the purpose of the reverse function on a VHF/UHF transceiver?

The 'reverse' function on a VHF/UHF transceiver temporarily swaps your radio's transmit and receive frequencies. Because a repeater receives on one frequency (its input) and transmits on another (its output), pressing the reverse button allows you to listen directly to the repeater's input frequency. Listening on the input frequency is a highly useful troubleshooting step. It lets you hear if the station you are talking to is close enough to be heard directly without the repeater. If their signal is strong on the input, you can suggest moving to a simplex frequency to free up the repeater for others. This function has absolutely nothing to do with reversing the radio's power polarity, changing its power output, or reversing the audio signal.