yeah basically what he said-------^
mortars can be very handy against hoard armies as not only can you stick them behind some tasty cover and still punish the enemy with out them getting shot up in the process but the miltiple templates and pinning effects can really mess the enemy up.
i'd suggest more than one squad of you want to do some damage though. you can get 2 from the command platoon, but for maximum effect take a heavy weapons platoon of 3 squad and a command section with another mortar for 290 pts and aim for the densist concentration of enemy troops
.