The Napalm Gun is a weapon that doesn't do a lot of direct damage. But by virtue of its effects, it's one of the most powerful weapons in the game.
The Napalm Gun launches a grenade at a high ballistic arc like a Car Grenade Launcher, but the grenades do very little damage even if they hit directly. What they do is to spread a pool of flaming oil that damages tyres and bottom armor.
The Napalm Gun is a direct strike at the mobility of your enemies. A field of Napalm Gun shots in your enemies' path will leave them slowed, crippled and easy meat.