What is the LXX and why is it so noteworthy that there is a Greek translation of the OT? Wouldn't it be better to reference the Hebrew original?
Before 1947, a good case could be made that the Septuagint represented a more ancient tradition than the Masoretic versions of the Tanakh. Since the Septuagint was produced before 132 BCE (and probably in the 3rd century BCE) and the earliest known Masoretic manuscripts date to the 10th century CE, the Greek translation might have fossilized an early rendering of the Scriptures while the Hebrew continued to be copied and potentially altered. To further support the primacy of the Septuagint, the early Christian and Hellenistic Jewish texts, tended to use that translation rather go back to Hebrew sources.
An interesting counter-argument arises from the observation that during the 600 years from Jerome's translation into Latin (the Vulgate) to the earliest Masoretic manuscripts, the Hebrew seems to have been faithfully copied. That suggests that Jewish scribes were exceptionally careful to duplicate their scriptures.
After 1947, the evidence changed. According to Wikipedia:
A few caveats are in order:
What remains therefore is a window into the community of Alexandrian Jews that produced the translation. For those who are interested in the development of Jewish belief, that can be extremely useful. N. T. Wright's book, The Resurrection of the Son of God, shows that the LXX translators chose to interpret key texts from Daniel and Ezekiel about the resurrection as literal rather than metaphorical. He argues from this that belief in bodily resurrection was more common in Jesus' day than other scholars assert.
The Septuagint (LXX) was the version of the Bible used by the authors of the New Testament. Therefore, the authors sometimes quote the Septuagint rather than the Masoretic (Hebrew) text.
The Masoretic Text of this quoted verse in Isaiah:
The difference is pretty striking.
Because of this, the translators of some versions of the Bible tend to use the Septuagint in the Old Testament.
The Septuagint text from that same verse:
Ultimately, the Septuagint is useful to help gain an understanding of why the authors of the New Testament used quotes that they used. It helps clarify understanding of how the Greek was used.
One of the most important aspects of the Septuagint is that it helps us understand how Greek was used by Jews in the 3rd century BC to talk about God and the Scriptures. This turns it into a valuable tool to look at the Greek of the NT and understand how to translate and examine it.
Here is an excerpt from a Christianity.SE answer that I provided to illustrate one use of the Septuagint in this manner:
IMO it is a mistake to consider the LXX too noteworthy. The NT authors quote from it under the inspiration of the Holy Spirit, however that should not be taken to imply that the LXX translation as a whole is inspired.
Yes, except when dealing with the NT quotations in question. And translators are making a mistake if they use the LXX as an authoritative basis for translating the old testament even for those passages quoted in the NT
Of course none of this is to say the LXX is not useful as it may shed light on translation issues, as mentioned in other answers.
The reason I most often use the LXX is to find the concept the NT authors were using. Yes, they wrote in Greek, but they were thinking Jewish thoughts. Many times, you can take the Greek words in the NT, find them in the LXX, and see what Hebrew words they translated.
For example, the word ecclesia is used in the NT in Matthew 16:18 and 18:17. Some argue that this means Matthew is written late because Jews in the time of Jesus would not use the concept of ecclesia. Patently false. If you check the LXX, you find the word ecclesia commonly translates qhl, which means "sacred assembly." And that means 2 things. 1) As the LXX was translated centuries before Christ, Jews were familiar with the concept of ecclesia and used it. 2) Matthew's use of ecclesia does not mean it is late.