What are the reasons why someone asks this question?

There are two types of people who ask these questions. First, they might be insecure about themselves, and they're not sure why you're their friend. They may also not value themselves, so they're looking for someone to validate them. Second, introspective people. They want to know what they're bringing to somebody, and they want to know what they can do more to make sure that they're meeting the needs of the other person. If you were asked this question, I would respond with genuine responses like, you're a great person and you're perfect just the way you are.