I never do such thing, my demons obey me because they consider that I’m worthy of being served and I keep working so they keep having that opinion of me.
The goal is to feel like you are all-powerful; to know that you have the authority and capability to command the spirits that you’re summoning. This doesn’t necessarily translate to actually being omnipotent in the sense of having absolute power over the Universe at large, because you’re right, if that was the case then why bother summoning a spirit in the first place?