What is spark plug gap for?

Coils may not have enough stored energy to fire, or in the least case, not enough energy to cross the gap, creating a miss-fire. Plug wires will break down due to the added resistance as the spark tries to reach ground. Rotor and Cap, as well as points, and the carbon bushing in the center of the distributor cap will show early failures. All of this is because the greater the gap and the higher the voltage requirement to jump the gap. Do not forget the gap between the rotor arm to the distributor cap too. A high performance rotor is a bit longer at the tip, allowing less spark loss or chance of spark scatter in the cap as the spark attempts to jump the plug gap.

Thanks for the info. I have a 1942 farmall h with a magneto and was considering opening up the spark plug gap from 0.25 to 0.30 because I just put in stepped head pistons and the motor has a slight miss and I was told to open up the gap to make it stop missing. Do you think this would be a good idea or not? Any other suggestions? Thanks!! Any ideas would be appreciated!