Why did Jesus tell the three Apostles to keep quiet about what they had seen at the Transfiguration and what is the significance of this silence in Mark's Gospel?

Jesus took Peter, James and John up into a high mountain, where his appearance changed, and they saw him talking to two strangers. Somehow, the disciples knew without question that the strangers talking to Jesus were Moses and Elijah. There is no evident theological or practical reason for the Transfiguration: if Jesus is the Son of God, he could have talked to Moses and Elijah at any time and any place he chose, and in any case those two could scarcely have told Jesus anything he did not already know. Read literally, the sole reason for this journey was to let the three disciples see Jesus talking to Moses and Elijah. Yet they learnt nothing from the experience and were even instructed to tell no one until he had risen.

Mark's Gospel was the first New Testament gospel to be written, and this gospel frequently refers to Jesus telling those who witnessed his miracles to keep quiet about what they had seen. It is as if Jesus did not want anyone to know about him or his mission on earth. Or, one could reason that the author wanted to be able to explain why no one had heard elsewhere about the miracles that supposedly occurred forty years before this gospel was written.

Mark originally ended at verse 16:8, with the young man telling the women that Jesus was risen, and they fled, telling no one. Until the "Long Ending" (verses 16:9-20) was added to Mark's Gospel long afterwards to provide the necessary resurrection appearances, the Transfiguration episode provided the best evidence that Mark could offer, of divine support for Jesus. Also, in the framework structure* of Mark's Gospel, the Transfiguration of Jesus and the voice of God from heaven were an essential counterpart to the baptism of Jesus and the voice of God from heaven.

The later gospels are known to have been based, directly (Matthew and Luke) or indirectly (John), on Mark's Gospel, but each writer added resurrection appearances that proved that God raised Jesus from the dead. The Tranfiguration was of less importance in those gospels, but still a compelling story of the divine.

Footnote
*Mark's Gospel is based on a parallel structure, which is a literary sequence in which an opening set of events is contrasted with another, parallel set of events that mirrors the first. Mark's Gospel as a whole consists of a parallel structure and then contains smaller chiastic structures, particularly around the last day and the crucifixion.