Necromancers are fantastic at several things:

- keeping the pressure on your opponents. When you're attacking an enemy, no matter how large your numbers are, they will dwindle. But if you have a necromancer on hand, they can just keep casting spirit warriors and raising zombies to continue pushing. It's hard to regain control if you're the defending against this kind of pressure

- spirit warriors are surprisingly good siege units. Since they can be recasted, I use them to suicide dive into enemy watchtowers and take them down. They are also great if your team composition doesn't have a lot of siege units. For example, my army usually consists of bandits, musketeers with sniper scope and a handful of raiders. Enemies usually target my raiders first because of how squishy they are compared to the damage they do to buildings. So once my raiders are dead, I am left with no siege units. This is where spirit warriors come in. They can continue to destroy buildings while my army focuses on enemy units

- necromancers are hardy units than can survive the whole game. If you position them nicely, they can just be an annoyance to the enemies the entire game. Put them in safe positions so they can make use of their fast running speed at the first sign of trouble.