Personally speaking, it is something that control our behavior and words. The essence of reason is to deny and doubt. Of course, I think it can be called judgment and analysis of things.
Reason allows us to make objectively optimal choices, especially when it comes to risky things. It makes us to pull back from the brink and cut our loss in time. Also, reason can even save our lives.
For instance, if someone irritates you, you get extremely angry and want to kill him with a knife. While your reason tell you that you can't do that. It's irrational. If you do that, there will be serious consequences. Your reason is telling the right and wrong of things and the loss you will get.
When we are in a period of emotional underestimation, especially depressed people, they are likely to commit a suicide. However, when reason gets the upper hand of depression, then things will get back to be normal.
