It's one of the key questions of the play.
For a long time, critics thought that the comic section in the very middle of the play was probably inserted by other writers to pad the play out - and that it probably wasn't the work of Marlowe. Yet over the last few years, it's become increasingly populr to suggest that Marlowe himself composed the comedy scenes - they do, after all, serve several important purposes within the work.
Firstly, they provide comic relief for the audience after the ominous legalistic deal-brokering between Faustus and Mephistopheles. Secondly, they chart the years of Faustus' magical powers before his eventual damnation, in a form not unlike a TV montage: but crucially, they show the time passing. And finally, they show the change in Faustus' worldly ambitions - meaning that, when the tone returns to more serious topics at the end of the play, there is a feeling that the play has slowed back down to real-time.
The scenes themselves also touch on key themes: greed, stealing (Rafe and Robin, the Horse Courser) and of course, the ridiculousness of the Pope and the hierarchy of Catholicism. They've received pretty mixed reviews from critics - but I thnk it's important to read the comic scenes carefully: and remember that comedy can always be serious.