When studying a text, I tell my students that nothing is accidental and that details are included or excluded for some sort of reason. Part of discussion is hypothesizing what that reason is (in each case or on the whole).
In the Torah, we see a set of details -- some narratives are detailed, others are dealt with only superficially; some people are explored at length, others (ostensibly just as important) are not; some laws are given explicitly and others are barely mentioned.
For example: the written Torah details the list of forbidden relationships, but provides no explicit detail of "work" forbidden on Shabbat. The Torah tells us how to behave with our slaves and animals, but not how to slaughter properly.
This is not a question of relative importance. Material provided in the Torah Sheb'al Peh is equally as necessary for the complete understanding, but is there a reason that certain material was put in one medium and some in the other?
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