Based on playing Civ I just assumed they were mostly valuable for their use in making gunpowder.
Food to feed your troops (and workers in armament factories) is a strategic military resource.
You have to either grow it yourself or import it. Artificial fertilizer allows you stop importing nitrogen (which could be blocked by embargoes) and make it locally, which would allow you to potentially grow food locally and not import it either (again, embargoes).