Barclay* tells us two reasons Jesus would have had the disciples accompany him. I always got the "witness" concept but hadn't thought about the other reason he advances. It makes perfect sense.
(a) They had been shattered by Jesus' statement that he was going to Jerusalem to die. That seemed to them the complete negation of all that they understood of the Messiah. They were still bewildered and uncomprehending. Things were happening which not only baffled their minds but were also breaking their hearts. What they saw on the mountain of the transfiguration would give them something to hold on to, even when they could not understand. Cross or no Cross, they had heard God's voice acknowledge Jesus as his Son.* (Do keep in mind that I like Barclay's insight into language and bygone customs, but his theology can be a bit wacky. That's not to say that I often don't find him inspiring. He can be. But just know that he should be read with caution.)
(b) It made them in a special sense witnesses of the glory of Christ. A witness has been defined as a man who first sees and then shows. This time on the mountain had shown them the glory of Christ, and now they had the story of this glory to hid in their hearts and to tell to men, not at the moment, but when the time came.
The Gospel of Mark
(The Daily Bible Series, rev. ed.)