When Faustus resolves to sell his soul to the devil in exchange for Mephistopheles, his decision to give it is not enough. Instead, Mephistopheles tells him, "thou must bequeath it solemnly / And write a deed of gift with thine own blood; / For that security craves Lucifer" (2.1.33-35). Basically, that means that Dr. F has to sign over his soul in a written contract. There's no way outta that one.
Faustus does as Lucifer asks, writing a contract that gives ownership of his soul to Lucifer in exchange for twenty-four years of having Mephistopheles around to boss. In legal terminology, this contract is called a "deed of gift," and it's much more than just a piece of paper with some bloody chicken scratches.
It becomes the symbol of Faustus's decision to serve the devil. And since some definitions of sin define it as just that—the decision to serve the devil instead of God—it also becomes a symbol of sin more generally.
This contract is doubly scary because it's very, very real. It's made of paper and blood, and Faustus and Mephistopheles can see it and hold it in their hands. So while Faustus's decision to serve the devil is not much more than an idea, this is an object that Lucifer can point to and say, dude, you belong to me.
It's easy enough to take back a decision by making a different one. Don't like the pair of socks you put on this morning? No matter—wear some sandals instead. But a written contract is unbreakable by law. Faustus's decision is put into document form to symbolize just how difficult, if not impossible, it will be for Faustus to take back even if he really wants to.