No, they heat the air, prior to induction.
With a petrol engine, so long as the mixture in the cylinder is correct, and properly compressed, it will explode when it gets a spark.
With a diesel, the air is compressed first, then the fuel is introduced. The explosion is dependant upon the air in the cylinder being at a certain temperature. If it is not at that temperature, then it simply will not explode when the fuel is injected. The flame plug makes sure that the air is at or above the right temperature - absolutely correct, mind not on the ball, as only air goes through the manifold - the fuel is injected