'Rashi': This is like "Ma'ase Choshev" (Shemos 28:6). They are professional weapons, like a dyke/bulwark and battering rams [used to topple a wall], like it says "b'Sachbulos Ta'aseh Lecha Milchamah" (Mishlei 24:6).

Malbim: Uziyah was a Chacham in calculations and physics, how to shoot heavy loads a far distance with a small force. Catapult rocks shot at a wall help very much in war. He also made such machines in the towers to shoot at the enemy. He became famous for this; such war strategies were not known beforehand.