The Septuagint was the first translation made of the Hebrew Old Testament into Greek. It was begun over two hundred years before the birth of Jesus. It was translated from a Hebrew Old Testament text-type that is older than the Masoretic text, from which most Old Testaments are translated today. This is sad, for the apostles had access to both the Septuagint and to the proto-Masoretic text that was in existence in their time. And they chose to quote from the Septuagint—not the proto-Masoretic text.
You have probably noticed that many of the Old Testament passages that are quoted in the New Testament don’t read the same in the New as they do in the Old. However, if you were using the Septuagint Old Testament, they would read the same.
For the rest of the article on the importance of the Septuagint go here.