Ok, sorry to sound like a FNG, but I just now realized the true potential of on-board mortars.
I was on defense, German, facing a British charge right up the middle through some trees. My position was about to be overrun, due to my poor positioning of my troops, and the demise of my supporting tank. Previously, I mistakenly had thought that my mortar could fire from anywhere on the map, using any commander's LOS, so imagine my surprise when it failed to happen, and the opportunities I had missed.
However, I hustled my mortar up right adjacent to the house the commander was spotting from, let loose some shells, and completely crushed the offensive right at the crucial moment. After destroying 6-7 squads, I shifted fire to the next concentration, and wreaked more havoc. This one even was pivotal in me winning the battle (just a Quick Battle) with a score of 82, as well as completely demoralizing the enemy troops.