Eliezer wanted to retain the gold crown on his tooth because it might be useful in helping he and/or his father to survive at some future point during their ordeal in the concentration camps of his Night. "It could be useful to me one day, to buy something, some bread or even time to live."
However, the torture of Eliezer's father became more than Eliezer could endure, and he finally gave permission for the gold tooth to be extracted. In exchange for the gold, Eliezer received a little more soup a few times and his father was saved from the beatings of the Polish foreman Franek who had wanted the gold and eventually obtained it. However, the Polish prisoners, including Franek, were moved out of Buna two weeks later, and any advantage the gold might have bought for Eliezer was gone.