Line it up against a rocket and the appeal is plain. A rocket costs 1,400 sulfur and waits on the explosives blueprint at a Level 3 bench; this bomb needs neither. The catapult, the bomb and the empty propane tank are all Level 2 blueprints, so a group stuck below Level 3 can still put a siege together. Whether a wall falls to fewer bombs than rockets is something to test on your server, not to assume.
Most of the craft time is gun powder. Start the powder queue days ahead and leave the bombs for the evening of the raid. The tank is counted as is: barrels hand them out, and once you research it a tank costs fragments and a little scrap, which beats waiting on loot.
Watch out
Don't spend propane on a wooden base. A firebomb costs a third of the sulfur and burns wood just fine.
