Chulent was a way that Jews set themselves apart from the Karaites. The Karaites, not ascribing to oral halacha, said that no food can be cooked evenly partially on Shabbat.
On the other hand, the lineage following the rabbis that now makes up today’s Jewish people, affirmed that indeed if the food was cooked by a certain measure before Shabbat, the rest can cook on the fire/heat source for the Sabbath.