Why do you use the Christmas tree to celebrate Christmas?

Because it's an ancient pre-christian pagan tradition, at first disapproved of by christians, then later co-opted by them. See Isiah 10:1-5 - the King James Version reads: "Thus saith the Lord, Learn not the way of the heathen.... For the customs of the people are vain: for one cutteth a tree out of the forest, the work of the hands of the workman, with the axe. They deck it with silver and with gold; they fasten it with nails and with hammers, that it move not.