I have thought about this problem, but I think there may be many reasons, including culture, customs and reality. For example, in China, since ancient times, people have always believed that the universe is round, and the circle represents perfection and beauty, so many beautiful things are round, such as the full moon, mirrors, wellheads, water tanks, jade vessels and so on. As the object of worship in agricultural society, food should be a very important thing, and food is the most important thing for the people. Therefore, the bowls and plates used to hold food are round, which has the inherent meaning of food worship.

In addition, from a practical point of view, circular objects should be more convenient to use, because there is no water chestnut, so it is easy to place and pile up on the desktop. The same area, square or water chestnut, it is better to save time and land. And if you hold it in your hand, it is also very convenient to hold it at all angles.

This is just my personal guess. Perhaps from the point of view of enamel technology, baking tray is more convenient. And this circle looks more beautiful whether it is put on it or filled with food and soup.

Later, many other shapes of plates appeared, including square, flower and fish. It should be that after the food is already abundant, people regard diet and cooking as a kind of culture and an affiliated pursuit, which is mainly designed for visual beauty.