Pls explain how early summoning spells are useful

[NTJedi]

Teraswaerto said:
Why do you say think Howl is so bad? The wolves come from all sides, even behind, and can kill enemy mages and disrupt their troop formations. Swarm only hits the front lines, and doesn't kill anything.

Howl costs 2 nature gems and requires 2 nature to cast. Howl provides very very few wolves which usually retreat after a few are killed.
Swarm is cheaper to cast, easier to cast, and the units will all fight to the bitter end not retreat unlike the wolves. Their flying works great for stopping the fleeing units as well. If you cast one Howl(2_gems) against one swarm the swarm will win and you'll have an extra gem.