Someone recently had a great question: “Why the three divisions of Weekly Torah portions into Torah, Haftarah and Brit Chadasha?”
The weekly Torah portions (Hebrew: Parashas) serve to cross-reference the Torah and Tanach ("Old Testament") passages with B'rit Chadasha ("New Testament") passages to reveal their seamless continuity, thus revealing God's continuous, ongoing disclosures from Genesis to Revelation.
There are 54 parashahs in all, one for each week of the year (including leap years), so that in the course of a year, we read the entire Torah (Genesis to Deuteronomy). During non-leap years, there are 50 weeks, so some of the shorter portions are doubled up.
The Haftarah consists of the portions from the Tanach (Writings and Prophets), and Brit Chadasha readings are from the "New Testament" scriptures all of which correlate to that week's Torah portion.
The reason for reading the weekly, consecutive Torah portions are, of course, to get to know YHWH and what He expects of His people. We read the Haftarah portions to discover what happened to YHWH's people AFTER they left the "wilderness." And we read the Brit Chadash (NT) portions to learn "the end from the beginning" and to see how it all ties in.
WHY do we have "Torah portions?" Basically, to keep everybody around the world "on the same sheet of music" all year long.
The Rabbis of old decided exactly how the readings should be divided. According to Wikipedia, "The division of parashiot found in the modern-day Torah scrolls of all Ashkenazic, Sephardic, and Yemenite communities is based upon the systematic list provided by Maimonides in Mishneh Torah, Laws of Tefillin, Mezuzah and Torah Scrolls, Chapter 8. Maimonides based his division of the parashot for the Torah on the Masoretic text of the Aleppo Codex."