Why do reciprocating aircraft engines have more than one spark plug?
Bold Method has a great post today about why aircraft engines have more than one spark plug per cylinder. There are a few basic reasons.

Reliability

Having two sparks plugs is more reliable. If one spark plug becomes inoperative for some reason, there's a second to provide the spark for the power stroke.

It should also be noted that the spark plugs are usually powered by dual, independent magnetos or, in some cases, an electronic capacitor discharge ignition. The redundancy of the ignition system adds to the reliability.

More Power

Having two spark plugs means the flame ignites from two points leading to more power per power stroke. This point is demonstrated during run-up. When the magneto check is done the engine RPMs drop when running on a single mag and set of spark plugs.

Even Combustion

Two sparks plugs and a two flame front leads to a more even burn of the fuel air mixture for a smoother running engine.

Prevent Fouling

Aircraft engines burn leaded fuel which can lead to getting lead deposits on the spark plugs. Having dual plugs leads to a more complete burn which can prevent the deposits from forming.