All the parts of the Torah are essential.
They are not just complementary or supportive of each other.
They also use different means, different languages, to say the same thing, whether it is Halakhah or Kabbalah, Mishnah or Zohar.
For example, the prayer book has this formula for performing a mitzvah: "To unite the Holy One, blessed be He, and the Shekhinah."
This is a kabbalistic formula.
And it signifies that this union of Divine manifestation is the same single purpose of all our actions, no matter which of the mitzvot are involved.
The scope of the Torah is always beyond any of its parts.
It is always the same and it is possible to approach it, to view it, from many different angles.
–Rabbi Adin Steinsaltz
From On Being Free, p.181, by Rabbi Adin Steinsaltz