Defensive gameplay and other tactics.

Often, when storming a town or simply engaging in battle with an enemy hero in an open field, I would defeat them but lose all my troops, effectively suffering a defeat. Using "Raise Dead" instead of "Resurrection," I would destroy the enemy's army, but since "Raise Dead" doesn't allow the resurrected units to remain alive in your army, I would end up losing my entire force...
In such cases, it's very helpful to hire a Necromancer hero in the nearest town, even if it's just with a single skeleton, and then use the "Summon Creature" ability to summon that same skeleton into the army of the hero who is storming/attacking...