The answer is faith. Their faith was counted to them for righteousness. Faith in the Word of God spoken. There faith however was under the cloud and in need of teaching aids. Seeing glimmers of the purposes of God but not fully grasping it. Even so...They believed God, that the Serpent's head would be crushed and that the curse of Death would be lifted. Everything spoken after that initial promise has that at its center. God sends the flood but preserves the promise of the seed by keeping Noah and his family alive. Shem is blessed to be a blessing to the whole earth, Ham will serve Shem and Japheth and Japheth will dwell in the tents of Shem. The promise of the seed should be seen in the line of Shem. Abraham is born of that line and the promise comes to him that through Him all the nations would be blessed. Hebrews says much to elaborate the Christ seed in this OT context. On and on the promise of Christ is drawn out and with each Covenant renewal there is a greater revelation, understanding that God taught the OT people on two levels through physical representations and heavenly expectations. Hebrews says that Abraham sought a spiritual city and Jesus says that Abraham looked to HIS day. If you blink you will miss it in the OT passages. Hebrews speaks that though the land in the OT was considered to be a rest, that Joshua speaks of another expected rest. Blink and you will miss it in Ecclesiastes, where the subtle term "under the sun" acts like book ends to define the book - that this life, under the sun and apart from the divine, is incapable of giving meaning to itself.
Or how about the tabernacle which is patterned off what Moses was shown in Heaven and this too was a picture of the promise "God with us" - the incarnation. God dwelt in tents because his people did. Later He would dwell in a house because God's people did in the land. And of course eventually in the flesh because his people did.
Even Nicodemus is reprimanded by Christ when He does not understand the OT in relation to Christ. “Are you the teacher of Israel, and do not know these things?" and then explains to Him that as the Bronze serpent was lifted up so must the son of Man. In other words, Nicodemus did you miss the point of what the Bronze serpent was teaching?
Before Jesus they were under a tutor to bring them to Christ, and they should have received the physical representations with faith for the heavenly expectations.
Its late, sorry if I rambled a little.