Who knows, why did King Tang of The Journey to the West let Tang Sanzang go to the Western Heaven to learn Buddhist scriptures?

It is said that there was a king of Jinghe who bet with a fortune teller on the weather the next day. At first, the dragon king thought it was a shoo-in. Unexpectedly, the jade emperor really ordered rain, and the length of time was exactly the same as the fortune teller's prediction. In order to win, the Dragon King changed the amount of time without authorization. As a result, he broke the dogma and was about to be beheaded. The executor was Wei Zhi, a minister of the Tang Dynasty. The Dragon King turned to Li Shimin, the king of the Tang Dynasty, for help. When the King of Tang wanted to interrogate the Dragon King, Zheng Wei was left in the palace to play chess. At 3 noon, Wei Zhi fell asleep and beheaded the Dragon King in his dream. The Dragon King thinks that Li Shimin has broken his promise and the ghost has haunted him. Fortunately, Guanyin came and repelled the dragon ghost.

It turned out that many ghosts could not cross over because of the serious killings in the southern government, so they sent Guanyin to Datang to look for Buddhist scriptures and came to the West to learn the three dharma books of Mahayana Buddhism.

Taizong was frightened, fell ill and died soon. Before he died, Wei Zhi gave him a letter, saying that he had relations with officials in the underworld and could ask him to take care of him. In the underworld, the official secretly changed the book of life and death, which enabled Taizong to return to the sun and took Taizong to visit the underworld. Emperor Taizong witnessed the plight of ghosts in the underworld, and after his resurrection, he held a Buddhist conference to vindicate ghosts. Xuanzang presided over the meeting. At the meeting, Guanyin also took a fancy to Xuanzang as a monk. She told the Tang King that what she is reading now is Mahayana Buddhist scriptures, which is not enough to surpass all sentient beings. She needs Xuanzang to go to the Western Heaven to get the Buddhist scriptures of Sanzang Mahayana. Therefore, the Tang king recognized Xuanzang as a righteous brother and gave him the surname Tang. Because there is Sanzang in Mahayana Buddhist scriptures, it is named Sanzang. Tang Sanzang thus embarked on the road of learning from the scriptures.

I suggest you read the original journey to the west.