Going into Mark 9 from Mark 8, Jesus has just announced in Mark 8:34 that anyone who wants to come after him has to deny himself, take up his cross, and follow him. Then in 8:35 he said that if you want to gain your life you have to lose it. The last words of Mark 8 begin to set up what we see in Mark 9. Having called people to come and die for his sake in 8:34–35, he begins to provide the motivation people need to be able to do that in 8:38, when he speaks of the Son of Man coming in the glory of his Father with the holy angels. More of that needed motivation – motivation to give your life for Jesus – is provided in Mark 9.
Mark gives his audience motivation to take up the cross and follow Jesus by showing him in transfigured glory (Mark 9:1–13), then he shows Jesus overcoming the failure of the disciples to cast out a demon (9:14–29), and he presents Jesus teaching on discipleship (9:30–50).