Question

One purpose of the dual ignition system on an aircraft engine is to provide for?

Answer 1

Answer: Redundancy

Explanation:
An aircraft is a system that cannot tolerate faults or errors because the consequences can  be fatal.
Therefore the ignition system is an excellent design candidate for redundancy.
Redundancy ensures that if the primary ignition system fails, a reliable backup ignition system will be available.

Another example of redundancy is the use of electric power generators in a hospital. They back up the primary electricity power supply because failure of electric power during surgery cannot be tolerated.

Answer 2

It is done to provide for better engine performance. It is important for two main reasons. One is that if one ignition system fails the other can for a time take care of it and hold its ground until you land or fix it. Another is that it is used for more efficient consumption of fuel and air which makes the engine work better.