There are two reasons why an interviewer would ask this question. First, they want to know what you expect for salary, flat out. They want to know, based on previous experience what you expect to be paid based on your level of experience and training. Second, they want to ensure that they can meet that salary expectation. If you walk in expecting double of what they are willing to pay, they will want to know what kind of negotiation can be made if they decide to hire you.