Why do we use a christmas tree

It's actually an old Germanic tradition for the Yule celebration. Fathers and sons would go out on the Winter Solstice and find the largest log to drag back to the home and that would become the Yule Log that would keep the family warm through the longest night.

In addition to the Yule Log, evergreen trees were brought into the home to give hope to families. These were hardy trees that not only lived through the harsh winters, but thrived. It gave people hope to see that somethng could survive such terrible weather anc climate.

The Christmas tree became popular among non-Germanic people in the mid-1800's when Queen Victoria of England married Prince Albert, of Germany. He brought with him the tradition of the evergreen. It was highly publicized, and many people wanted to be like their royals by having a Christmas tree as well. It became a sign of high society and culture to have a Christmas tree in the home.

I agree with the above post.  As far as I know, everything there is correct. 

 In the United States the Christmas tree began to be used more by almost everyone after the devastating Civil War ended.  So much family sadness was caused by that war, that people were anxious to have "hearth and home" in tact once more.  Also, before the war, most children were seen as additional labor hands in the normal family.  They were not coddled after they started to walk. 

The war taught families to treasure their children and small toys were attached to the trees to treat the children. Lady's Gody book printed a picture of Queen Victoria's table top tree and many American families tried to duplicate it in their own small way.  Children began to be thought of as the treasures they really are.  Thomas Nast drew a picture of St. Nicholas which pretty much survives today and the Industrial Revolution brought the comman man's ability to purchase "toys" for their children.  Clement Moore's poem helped promote the idea of providing a special event for the children. 

Most of our placing of any evergreens around our homes at Christmas time was adapted/adopted from the pagan religions by the Christian Church.