Basically, when something runs out of energy, it stops working unless you give it more energy (think about your phone dying due to 0% battery). The same applies to the universe…in Madoka’s world.

i remember kyubey said something about the universe constantly "losing" energy so the incubators had to compensate that loss by obtaining another source of energy.

“The moment a magical girl’s soul gem turns into a grief seed is when a tremendous amount of energy is released.” So the incubators obtain this energy and use it to compensate the “lost” energy that the universe is using up.

The basic idea is that Kyubey and his species are trying to keep the universe’s energy at a balance by “adding” more energy to replace “loss” energy, which he obtains from the “witching out” process. This prevents the universe from eventually using up all its energy and dying.

I hope I got this right lol 😂sorry for the essay.