Every time you interview, your main purpose is to convince the employer to hire you. The job search is a sales process. The hiring manager (your future boss) is the "customer," and you are both the sales rep and the product. Why should they choose you over all the other "products" out there?
The dreaded interview question, "Tell me about yourself," stumps a lot of folks. People of all ages and experience levels often fail to answer this one correctly, in a way that conveys meaningful information to the interviewer—information they will actually use to consider your candidacy.