In Scene 9, Robin and Rafe again provide some comic relief while making a point
In two previous scenes Faustus has been treated to a parade of the Seven Deadly Sins and, with Mephastophilis's help, has played low pranks on His Holiness, the Pope
In Scene 9, Robin and Rafe use magic to tease and torment a lowly wine merchant, but before Faustus pranked the Pope, though the subjects of these pranks differ in social rank, the pranks themselves are equally mean-spirited & petty
Scene 9 is meant to highlights how Faustus has knowledge and power, but his instincts for its use are no more noble or moral than the instincts of a couple of ridiculous lowlifes
In Scene 9, Mephastophilis punishes Robin & Rafe for their audacity, which is both amusing and a reminder of his role of demonic destroyer as he transforms them into an ape and a dog to emphasize their lowliness and foolishness