Preventing a crash.
I use VOLT-CHECK'S in most of my planes usually mounted in the cockpits.
One saved my World Models GS P-51, after tuning the Saito 180 for the maiden flight, I was ready.
Moved the plane to the hot-pits for start up, ready to taxi and noticed the red light in the cockpit, battery near dead, had one dead cell.
Volt-Check saved the plane.