Haber on the A award of the Nobel Prize at Stockholm ... soon made it clear that the basis of an important technical process could be found in ammonia synthesis, and further work was undertaken ...

Iron is a catalyst for the reaction. It increases the rate of the reaction without being used up in the reaction.

The temperature chosen is a compromise between yield and rate. Using a catalyst in the Haber process means that a lower temperature can be used whilst keeping the rate of reaction high.