>>2294086The universe is still destined for heat death, someone needs to prevent it.
Remember, in the final scenes where Homura is relating the story of the previous universe to Kyubey, he remarks about Witches being incredibly interesting, due to how efficient they are for collecting energy. Compared to those cubes or whatever she was feeding him gained from the Wraiths.
Magical girls still exist in the new universe. They simply don't turn into witches when they die, rather going to Meguka heaven.
