A backyard chef turns on the propane gas to a barbecue grill. Even though the reaction between propane and oxygen is spontaneous, the gas does not begin to burn until the chef pushes an igniter button to produce a spark. Why is the spark needed?

There is an activation barrier that must be overcome to begin the reaction.

Explanation:

The spark adds energy to the propane/air (oxygen) mixture.

The spark provides the energy to the propane/air mixture needed to overcome the activation energy barrier to start the combustion reaction.

Once started, the combustion reaction itself is self-sustaining since it is exothermic and generates sufficient energy to overcome the activation energy barrier.