What is the function of the painting the last supper?

Answer 1

There are hundreds if not thousands of paintings depicting this New Testament scene.

Assuming you mean the famous work by Leonardo da vinci it is a wall painting (a mural) in the dining room of a convent. Every time the monks sat down to eat they would have the feeling to be at the table of Christ.

Answer 2

Artists in da Vinci's time, and Leonardo himself were commissioned by wealth patrons, sometimes even the Pope, to create works of art. The artist also did not have the freedom to choose a subject. The fresco of the Last Supper by da Vinci is simply an illustration of a very important moment in Christianity. Many artists have painted this theme because of it's importance in Christianity and history.