It isn't. There are two types of decision in which rationality only contributes to one, the other being emotional. Rationality is excellent for considered choices based on evidence, known desired outcomes, and probable consequences. It's not so good for problems which require immediate action, for example when driving, where rationality comes second to responsiveness.
Rationality is one of the later functions evolution has gifted us, however it's restricted by the freedoms and time available to us