CVs are often sent by job applicants in response to a job advertisement as a way of providing information to the potential employer. You have no control over the content of a CV and these can therefore vary in length and style. CVs may well omit information that you feel is necessary to assess the capabilities and experience of each applicant. This stage, although the earliest stage of communication with an applicant, should be the point at which you begin to sift out the good applicants. Insufficient, unordered information does not allow you to do this easily.
Application forms, alternatively, are a probing tool providing a standard format for applicants to give you the information you require. Applicants are guided in what information to write about, and are also guided in how much importance they should apply to each section by how much space is given for them to write.
Application forms should be sent to anyone who has contacted you expressing an interest in applying for your vacancy, even if they have already supplied a CV.