It validates many of the basic flaws people see in themselves. It's relieving to learn that there are other people who struggle with similar problems as you do. On the other hand, it allows people to better realize their strengths.
Before reading about ISTJ I felt I was frightful, spiteful and unwanted. After reading about ISTJ I learned I was steadfast, loyal and basis of whole damn civilization as we know it. Made me feel good about myself. It's easier to be proud of oneself when you know your strengths.