There are two ways of answering this so I will do my best to answer both.
1) It is a traditional Jewish sign of grief or mourning. It is called the Kriah (Kree-AH).
Genesis 37:34 34 Then Jacob tore his garments, put sackcloth on his loins, and mourned his son many days.
2 Samuel 1:11 David seized his garments and tore them, and so did all the men who were with him.
The second was so that no one could say that man/men had done it. It could have been done by several men, making a cut in the middle of the bottom and then pulling on the lower corners. Being done fro9m the top to the bottom, it had to be done by God.
Take a look here for greater insight:
http://www.jewishencyclopedia.com/articles/7689-high-priest
The High Priest is critical to Israel, but Jesus arrived in apostate Israel. Prior to Jesus with who did the Jews work out their salvation? Who is John the Baptist?