Our need for purpose, both of individual actions and our life as a whole, sprouts from our rational nature. After all, Purpose is a statement that articulates the reason behind an action. As conscious human beings, we do things not just out of subconscious instincts, but through conscious intelligent reasons (Purpose is conscious).
The rational necessity of purpose is specially true when using it as a convergence tool (purpose is a convergence tool) to organize one’s life. Arriving at an organizing principle requires a lot of rationalizing (find your purpose through divergence–convergence).