When I first started playing, mortars were a unit that I often lost before I had even used any of their shells and never used them to their ability. Now, most of mine usually survive my battles hiding out after using all their shells(Unless there victims of a well placed artillery barrage). Once you get the HQ spotting down, they can greatly assist your infantry squads trying to gain some tough ground on well defended areas. Your enemy may be too close to your squads for an artillery strike to come in and possibly take out some friendlies. You have so much more control on where you want the on-board mortar shells to land and can make those tough MG units break and run(well, walk)after a few direct hits. As a receiver, I can often ride out a 81mm barrage pretty well on a whole, but that pesky mortar unit pinpointing on one of my units can drive me nuts especially if I can't tell where shells are coming from.