The Orc arsonists only had a range of about 30 metres, but if they were close to the city wall, it would be enough to attack the city defenders above. The smoke and stench were also very useful for getting close to the city wall. The reduction in accuracy of the shooters and archers above the wall meant the increase in the survival rate of their side.
Anyone who had a working brain would not resist trading for some of these quality products that were available at an affordable price.
The second special creature option was the Uruk-hai rushers. They had terrifyingly high defence thanks to the full plate armour they wore. They could also temporarily buff themselves up with Sauron's divine power, enabling them to sprint forward at high speed while ignoring the effects of the terrain during the sprint. Once the city was breached, they would obviously be the main force to quickly force their way through.
