They're very handy for goading defending enemy forces to attacking your army, rather than you having to chase after them, which will bring them into range of your other artillery pieces. I've personally found the mortar more useful than the rockets, as mortar has a much higher chance of hitting its targets, plus different ammo loadouts.

They're also good for taking out enemy artillery if you've formed up your forces out of range of their guns, and they're using static emplacements.