We do not know why Jude chose to use a story from The Assumption of Moses as an illustration from which to make his point. Maybe the people to whom he is writing were quite keen on speculating about the angels and so he wanted to use an illustration that they would find appealing. In this case his use of this story is similar to what the apostle Paul does in Athens (Acts 17:28–31) or when writing to Titus (Titus 1:12).
9 But when the archangel Michael, contending with the devil, was disputing about the body of Moses, he did not presume to pronounce a blasphemous judgment, but said, “The Lord rebuke you.”