IceBear said:
Yep, its to keep a single demon/devil from summoning an army at an exponential rate and taking over the Prime Material. Its allows devils/demons to be in the world without being able to singlehandedly destroy it.
It also keeps summoners within power (how do you rate the CR of something than can summon something, that summons something, that summons...). And it also prevents summon spells from getting you free wishes and the like.
