Why did Guan Gong and Zhang Fei stick to the door leaf?

I hope it helps you: It's not Guan Gong and Zhang Fei, but Qin Qiong and Wei Chijingde. Their main function is to ward off evil spirits and avoid ghosts. There is another story about this: After the Xuanwumen Rebellion, Li Shimin massacred Li and Li Yuanji's family on a large scale, even the babies were spared. Later, Li Shimin was afraid that his brother's ghost would kill him, so he sent Weichi Gong and Qin Qiong to guard the door. But later, because they were too old to serve for a long time, they had to draw portraits and hang them at the door, which became the origin of future goalkeepers. This legend was later quoted by The Journey to the West. Journey to the west: Jinghe Laolong near Chang 'an made a bet with a fortune teller and broke the dogma. The Jade Emperor sent Wei Zhi to supervise the execution of Lao Long at three noon. The day before, Lao Long asked Tang Taizong to intercede for him, and Tang Taizong readily agreed. The next day, Emperor Taizong announced that Wei Zhi had entered the DPRK and left Wei Zhi to accompany him to play Go. Unexpectedly, at three o'clock at noon, Wei Zhi dozed off and dreamed of beheading the old dragon. Lao Long resented Emperor Taizong's treachery, pestered him, and made trouble in the palace every day, which made the six gods of Emperor Taizong uneasy. Knowing that the emperor was afraid, Wei Zhi sent two generals and Weichi Gong to guard the palace gate. Sure enough, the old dragon dared not make trouble. Emperor Taizong remembered their hard work of guarding the door at night, so he asked the painter to paint their portraits and stick them at the palace gate. The result still worked. As a result, this move began to spread among the people, and Weichi Gong became a keeper. "The Romance of Sui and Tang Dynasties" contains: For a time, Li Shimin was in a bad mood. When he sleeps at night, he often hears bricks and tiles thrown outside the bedroom and ghosts screaming, which makes the harem restless every night. He was afraid and told the monarch and his subjects about it. General Qin said: "I have been an officer all my life, killing enemies like melons and collecting corpses like ants." Why are you afraid of ghosts? " I am willing to guard the palace gate with Jingde. "Li Shimin agreed. Nothing happened that night. Since then, I have put two generals on guard every night. Later, Li Shimin suspected that the two men had worked hard, so he ordered the painter to draw the full-length portraits of the two guards hanging at the door, and the evil spirits disappeared from then on. If you are good on earth, you will follow, so the gatekeeper has spread to the people. Up to now, people have always attached importance to the images of Qin Qiong and Jingde.