Sirach is the last of the wisdom books in the Catholic canon of the Old Testament. As such, it may be regarded as a massive summation of the Israelite wisdom tradition–a meditation on the entirety of Israel’s Scriptures from the perspective of “wisdom” (Hebrew hokhmah), the practical knowledge of righteous living. Because Sirach provides such a useful digest of the moral teachings of the Jewish Scriptures, the early Church used it extensively in catechesis and moral instruction, so much so that it came to be known as “Ecclesiasticus” — that is, “the Church’s little book”.
….despite the eventual rejection of Sirach by medieval Judaism and later by Protestantism, it was, in fact, regarded as Scripture by a substantial number of Jews during the Second Temple period and beyond. It was this latter view that the early Church inherited; not only was Sirach viewed as inspired by the majority of Church Fathers, but it was (as stated above) one of the most popular books in the Old Testament, generating a remarkable amount of commentary.
From “A Catholic Introduction to the Bible: The Old Testament” by Bergsma and Pitre.