Why? Here are some reasons:
- While the OT provides much of the content background for the NT (it provides the bullets), the Intertestamental Period provided the lens through which that content was filtered and read (it aimed the gun).
- We tend to read the NT out of the blue, as a contextless text that dropped out of heaven (or rather, we are unaware of the way our modern church context informs our reading of the NT texts). Once we are aware of the historical and literary context of the first century, it becomes increasingly difficult for us to read the NT in a timeless bubble (aka, what our Sunday School teacher told us).
- For those who have grown up with a certain kind of fundamentalism, there are strategies and techniques we are taught that probably don't make sense if we could step back and get some perspective. A reflective person begins to realize this when they read non-canonical writings and increasingly sense a contradiction in his or her method. The drive for consistency will gnaw at them.
- In short, the writings of "early Judaism" inevitably increase the Bible reader's objectivity and sense of historical consciousness, whereas Bible courses can sometimes merely confirm a person's initial biases (or those of the teacher).