Rationality is what enables humans to create norms for the avoidance of conflict over scarce resources. Norms allow for peaceful cooperation between people in a universe of scarcity. The validity of any proposed norm is determined through reasoned argument by the logic of argumentation itself. Rationality is the necessary tool in this endeavour.

Although norms are developed and validated in the course of rational argument by those engaged in argumentation, these norms apply to all humans, not just those doing the arguing. Norms are universal for two reasons:

The logic of argumentation entails universal applicability of norms to all those who are in principle capable of agreeing them.
All humans are in principle capable of agreeing to norms at some point in their lives.

These norms do not apply to animals, since animals are not capable of agreeing to or upholding norms.

In short, humans need norms in order to avoid conflict and rationality is the tool used by humans to create and validate norms.

This is how the role of rationality is understood in the theory of inherent rights.