Proton mortars were a long-range ground-to-ground weapon used during the Clone Wars. They fired a mortar shell in an arcing trajectory that would explode when it hit the ground, useful for dispatching groups of enemies or firing up and over fortifications. They were used primarily for shelling enemy defense from afar before sending in an attack force.
The AT-XT walkers of the Grand Army of the Republic were equipped with proton mortar launchers.