When a gentile is already circumcised and wants to convert to Judaism, blood has to be drawn from his penis. Why is this? What's the point?
The drawing of blood, in the context of circumcision, forms a blood Covenant with God. As part of the conversion process, the person needs to join this blood Covenant, just like all other Jews who are part of this Covenant. If a person had their foreskin removed in the past, then the way to join this Covenant is by drawing blood from the same area where the circumcision would normally have been performed in the context of conversion.