The writer of Hebrews starts the book off with, The "SON [is] ... the HEIR of all things" (1:2 ESV). Sonship means heirship. His goal in the first two chapters is to show that Jesus was far superior in rank and glory to the prophets or the angels.
God has a great DESTINY for man. Man was created to be God's joint-heir with Christ, a SON or a DAUGHTER of God.
Jesus became a man to become the FIRST MAN TO LIVE AND RULE AS A SON (vv. 6-8).
Jesus became a man in order to DIE as a man so that He could "taste death for everyone" (v. 9 ESV).
Jesus became a man, not only to pay for man's sins, but to "[bring] many SONS to glory" (v. 10 ESV, emphasis added).
Jesus became a man in order to make us into His brothers and sisters, in HIS IMAGE.
Jesus became a man in order to DEFEAT SATAN for His brothers and sisters.
Jesus became a man in order to become EXACTLY like His brothers and sisters so that He could become a "merciful and faithful high priest" (v. 17 ESV).