Why is Reason Important?
Reason (or rationality) is a tradition about how to think properly. It tries to avoid bias and find the truth whether we like that truth or not. It avoids superstition, magical thinking, parochialism, faith, hardheadedness and whim. Reason requires people be open to changing their mind. Reason also rejects the idea that authorities can or should tell us what the truth is. Instead, we should judge ideas ourselves, and judge based on the content of the idea not based on the person who said it. Even if I am the person who said an idea, and I have a PhD, that doesn't count for anything, it could be mistaken and should be judged on its merits.
Reason is not about which ideas are true. It's a meta-tradition. It's about how to treat ideas, and how to treat disagreements. You aren't unreasonable or irrational if you believe something false. I'm irrational if I believe something false and justify it with "because I said so" or "look at these certifications on my wall". With an attitude like that, if I am mistaken I will never find out and never improve. What violates the requirements of reason is assuming I know the truth from the outset. Doing that cuts off the possibility of learning I am mistaken or learning a better idea.
One of the core ideas of the reason tradition is that the truth is not obvious. People make mistakes. Even when we're really sure of ourselves, we still might be mistaken. And this isn't just a technicality. Mistakes are common, and we should be aware of them all the time. In a disagreement, a follower of reason thinks, "Maybe this other guy knows something I don't. Mistakes are common, so I may have made a mistake." So followers of reason try to learn from each other, and offer criticisms, arguments and reasons for their beliefs. This is not an easy process, but through it sometimes we learn something or correct a mistake.
Reason is all about correcting mistakes. We need to approach ideas in such a way that if we're wrong we will find out and improve. We can't ever get guarantees, but we can do a good job of it.
Reason rejects authoritative or privileged sources of knowledge. Accepting them would mean not using your own judgment. We need to use our own judgment to spot errors and try to do something about them. Leaving our judgment out of our approach to knowledge, even just sometimes, is taking away a possible source of error correction.
Reason is used in all fields of human endeavor that make progress. For example, the scientific method is a kind of reason. The scientific method is a meta-tradition that doesn't tell us what to think directly, but instead tells us about how to seek the truth. The physicist Richard Feynman said that science is about how to avoid fooling ourselves. In other words, the scientific method is designed to make it hard for mistakes to be repeated indefinitely. By following the scientific method, it's harder to make mistakes and repeat them, even if you want to (due to a prejudice, say). That's because the scientific method says important experiments should be double and triple checked by different people, and scientists should criticize each other's work, and scientific controversies can never be settled by working out who has a more prestigious degree or higher IQ score. The scientific method gives guidance about what to do to avoid and fix errors.
Mistakes can hurt people. Mistakes make things go wrong in our life, and not work out like we wanted. False ideas can lead to everything from economic recessions or wars to losing money with a gambling habit or being late to a meeting. Making fewer mistakes means being happier and meeting more of your goals. Reason is important because it can help with this.