Why do you need external validation of what you know? Is a test going to demonstrate that you know something? Is it going to demonstrate that you know what you

Why do you need external validation of what you know? Is a test going to demonstrate that you know something? Is it going to demonstrate that you know what you need to know, or what the person who designed the test thinks you need to know? And all that aside, what use is the test score? Is that something you'd put on a resume?