Mailing queries are, of course, only useful as on-site or embedded questions. You can use this question type, for example, for the promotion of your newsletter. Or you can ask for the email address at the end of a survey to send your customers a voucher as a thank you for their response.
Our recommendation: The question should not be placed directly at the beginning or in the middle of a longer survey. Without added value, visitors are reluctant to provide their email address. They are more likely to abandon your survey if you start immediately with the mailing request.