Well, so that my armies would still be on the rise even when I have dreadful casualties. It has a nice psychological effect don't you think, for you as well. Like me killing your mages and SC's but your army size not dropping due to the Soulstone and the free spawn.
Although, I've been under a lot of sieges these last few turns and alas those mages of mine don't reanimate under siege so my army size keeps still.