What's the significance of that?
The serpent in the garden?
The snake is Satan whom God told Eve that her seed shall one day bruise it’s head. I guess I could have added that. Mybe I’ll amend it one day.
Snake represents sin, which Christ became.
Wasn't it because God sent fiery snakes to bite people and kill them ? Or did you mean why did God pick snakes to begin with ?
A snake is a figure of evil in the bible. Yes I know people have perfectly tame pet snakes, and that’s not what is being talked about here.
The snakes were sent among the disobedient Hebrews, it looks to me, as an object lesson in the peril of inviting evil into one’s life and the need to deal with it. This problem will snowball unless and until taken care of. Evil has to be removed if permanent destruction is not to result.
The bible clues us in that “Jesus became sin” when He died upon the cross. That does not mean that Jesus did something He was not authorized to do. Rather, His spirit took our part and lifted our burden of sin, taking the wrath of God the Father upon that sin as He did so. And so our sin, lifted by Christ who had been incarnated as Jesus, was condemned on the cross.
And so the serpent lifted on the pole by Moses became a figure of the sacrifice which was to come. A serpent, because it represented evil being dealt with. A metal figure, rather than one of the actual serpents, because it was a substitute.
In a world filled with figures, care needs to be exercised since the devil can lay his falsehoods upon us about what the figures’ rightful use are. Later on, there was shameful worship of that same metal snake.