Some advantages of questionnaires:
The responses are gathered in a standardised way, so questionnaires are more objective, certainly more so than interviews.
Generally it is relatively quick to collect information using a questionnaire. However in some situations they can take a long time not only to design but also to apply and analyse (see disadvantages for more information).
Potentially information can be collected from a large portion of a group. This potential is not often realised, as returns from questionnaires are usually low. However return rates can be dramatically improved if the questionnaire is delivered and responded to in class time.
There are two different types of questions that survey researchers use when writing a questionnaire: free-response questions and closed questions.  Free-response questions are open-ended, whereas closed questions are usually multiple-choice.  Free-response questions are beneficial because they allow the responder greater flexibility, but they are also very difficult to record and score, requiring extensive coding.  Contrastingly, closed questions can be scored and coded much easier, but they diminish expressivity and spontaneity of the responder.