Dick and Robin are the town troublemakers, the class clowns. They get up to lots of mischief when they ditch their duties as stable boys for some rather misguided attempts at practicing the dark arts with Faustus's conjuring books.
In the stage directions, Robin is designated as the "clown," which probably just refers to his role as comic relief for the play. And these two do totally lighten the mood: Faustus has just sold his soul to the devil. That's what he does with his magic skills.
Dick and Robin, on the other hand, are busy scheming about how to get booze without paying a penny for it, or calling Mephistopheles to help them avoid punishment for stealing a cup from the local tavern. Yeah, um, perhaps not the loftiest goals in the world.
What's most surprising about Dick and Robin is that they actually succeed in calling Mephistopheles. Granted, he's not happy about it and turns them into a dog and an ape to express his displeasure, but these two have still demonstrated a surprising amount of magical abilities.
If they were they willing to sell their souls to the devil they, too, might be able to be as powerful as Faustus (and get an awesome vacation thrown in for a steal). In a way, the fact that Dick and Robin are so close to glory makes Faustus's accomplishments seem slightly less amazing. But it also highlights what distinguishes Faustus from the pack: he's got lofty goals. He's not in it for the booze. He's in it for power. And that's what separates the men from the boys.