What it shows depends on the panel. The good ones show current through the plug. If you turn up the knob and see no current you have:
1. dead battery
2. loose connection
3. dead plug
Once you know what current the plug of YOUR choice typically draws for the heat you want you can set it for that current.
The glow stick can't be set. Freshly charged it may give more heat than you want and when the battery weakens it may not give enough to fire the engine. The knob also lets you give a little extra current to help with a flooded engine.