It flies in the face of His commands not to make graven images. At first glance, it might seem as if they were worshipping the image, although that isn't the case.
Perhaps the image of the snake was intended to remind them of the Garden of Eden and Adam and Eve's first sin. Perhaps it was to look forward to Christ on the cross, where all who look to Him would be saved.
Perhaps it was like many of the instructions they received in the desert, without any obvious logical or scientific explanation, simply because God said so; although I believe all the "why" questions we have will one day be perfectly clear.
I often wonder why God allows bad things to happen. Or why He chooses to intervene miraculously in one situation, but not in another. I live by faith trusting the goodness of God that I see throughout scripture is the same yesterday, today, and forever.
John 3: 13-15 "No one has ever gone into heaven except the one who came from heaven—the Son of Man. Just as Moses lifted up the snake in the wilderness, so the Son of Man must be lifted up, that everyone who believes may have eternal life in him.”