Why do you eat jiaozi on the solstice in winter?

It is said that people eat jiaozi every year on the winter solstice of the lunar calendar to commemorate the "medical sage" Zhang Zhongjing.

There is a short story about Zhang Zhongjing, a "medical sage":

When Zhang Zhongjing retired to China in his later years, it was winter, with heavy snow and biting wind. People along the road were hungry and cold, and many people's ears were frozen. Zhang Zhongjing was very sad when he saw it, so he asked his disciples to set up tents and cauldrons on the solstice in winter to treat people with frostbite.

Zhang Zhongjing kneaded this semi-medicinal and semi-edible thing into the shape of an ear and named it Jiaoer. Later, it became jiaozi. Eating jiaozi on the solstice in winter is a good thing for Zhang Zhongjing, a "medical saint". There is also a folk song, "Nobody cares about frozen ears on the solstice in winter".

Although we no longer use jiaozi to treat frozen ears, jiaozi has become the most common and favorite food for people.