The preliminary or ngondro practices are so important that it has been said, “The preliminaries are more profound than the main practice.” Well, why is this? It’s because if you do the main practice without suitable preparation it won’t work, which isn’t profound at all. On the other hand, if you do just the preliminaries properly and never get to the main practice that will still work. You’ll still gain realization. So that is profound.
[From a teaching on Essence of Wisdom: Stages of the Path, Part 8 by Lama Tashi Topgyal. Translated by Lama Yeshe Gyamtso.]