In the good ol' days, some of the single stick radios had a feature called auto-trim, and this button is commonly used for that type of feature. The good news is that auto-trim is a feature in OpenTX, so you can do the same.
Other uses are pretty much anything you would like it to be. It could be used for a trainer switch, a camera shutter, landing gear, etc. OpenTX allows for toggling via a single switch.
For wiring, you will replace one of the switch inputs your radio already has. If using it in an X9D+ or X9E it can be wired into the extra switch locations on these radios, so you do not loose any of your existing switches. Most people do not use their momentary switch on their radios, so they will wire into the button there.
Hope this helps.
