If, instead of the Bible, you were studying an anthology of English literature, what would be the point of turning to Shelley to explain Shakespeare, or saying "Keats cannot mean this because Wilde says that"? You can compare texts within a common culture, notice literary influences, coincidences, agreements and disagreements... but each text would ultimately have to stand by itself and be read for itself. Asking what the anthology "teaches" would be a pretty stupid question.
Well, the Bible may be just that. An anthology. Providing for fascinating yet often misleading intertextuality. When you read a particular text such as the Gospel of John you should make sure you don't read a foreign notion (of "g/God," for instance) into it.