Why A repeater listens on its input frequency and retransmits on its output frequency, and your radio normally does the opposite: transmit on the input, receive on the output. Pressing reverse swaps those two, so your receiver tunes to the frequency the other station is actually transmitting on. That lets you hear the distant station's signal directly and judge whether you could work them simplex, or check whether the repeater is even needed.
Watch out Listening on the repeater's output is just normal operation, which is what the radio already does with reverse turned off. The power-related choices describe what a high/low power switch does, not the reverse button.
Reverse = listen where you'd normally talk. If you still hear them in reverse, try simplex.