Mortars fire projectiles at high angles...they have limited range, but their high trajectory makes them ideal for hitting targets in defilade i.e. behind a hill or wall...the idea with siege mortars is not to destroy the walls, but what's on the other side of the wall...while siege cannon blast away at the walls or works of a fortification trying to cause a breach, the mortars are raining down on the defenders, causing casualties, fatigue, and destroying infrastructure...
Therefore I don't agree with the term "mortar bomb"...
Gandalph had knowledge of gunpowder to make fireworks, so I assume Saruman did too... it's only one small step from firecracker to bomb...you just make it bigger, pack it tighter...I always assumed the Uruks dug under the wall at Hornburg and placed a bomb manufactured under Saruman's supervision...