The Holy Spirit may have wanted to leave us the Letter of Jude so that we could appreciate the Gospels and other writings of the apostles better. We would have to be familiar with the literature of those days to know how complicated and incredible religious books were, both the books of the Jews and of the pagans. In comparison, the Gospels and Paul seem to belong to this century.
This letter which is attributed to the apostle Jude Thaddeus was written, in fact, about the end of the first century. It denounces the false teachers like those mentioned in the letters to Timothy and Titus.
Nevertheless, the comparisons and the examples which are used come from the Jewish books of the time. The Church had not yet defined which books were inspired by God and were part of the Bible. Besides the Old Testament, Christians used the religious literature of the Jews (for example, the Book of Enoch, the Testament of the Twelve Patriarchs, the Assumption of Moses). So there are many legends concerning ancient times which we find in this letter. In this literary form which seems rather antiquated to us, there is a strong call to preserve the integral faith of the apostles, which at the time was a serious concern of the Church. Therefore, a few years later, the author of 2nd Peter copied part of this letter.