Why is all the family tree from David to Joseph mentioned in Bible?

If Jesus is **not** the son of Joseph, but the Son of God, why is it said that Joseph is a descendent of David? If Jesus is the Son of God and the son of Mary, then the man who took care of Mary and her baby could be any man (a descendant of David or not). Hypothetically, if Mary would have been engaged with another man than Joseph, that wouldn't change Jesus, so why does it matter that the "adoptive father" (Joseph) was a descendant of David? In Matthew 1, it says that Jesus is the son of David, because Joseph is the son of David, not because Mary was a daughter of David, as it is stated clearly.