Importance is never an absolute thing. While it would be obviously important for someone who is in the process of studying for a PhD in English Renaissance drama to know Marlowe's work, there are many people who can live perfectly happy and fulfilled lives without ever hearing of, much less studying, any work by Marlowe at all.
The first reason that Dr. Faustus is important for students of literature is its position in the history of English literature. Based on a German tale, Dr. Faustus was first performed in 1594 and first published in 1604, making it among the earliest of Early Modern English dramas. It also influenced Goethe's Faust, and thus is important in terms of its subsequent history.
From a religious and moral point of view, the play is a fascinating study in the psychology of damnation. Dr. Faustus is not an atheist; he believes the Bible to be literally true, and yet still agrees to sell his soul to the Devil for temporary satisfaction of his desire for knowledge, luxury, and power. Thus one interesting way to read the play is to think about the psychology of what causes us to sacrifice long term happiness for short-lived pleasures. The play is particularly poignant not just for the magnificent writing, but for the way that one can see how Dr. Faustus' bargain with the Devil corrupts him, from someone wise and intelligent to a sort of buffoon. He becomes one of the damned before the Devil actually arrives for his soul.