Privacy laws, such as the GDPR (see Article 7 - "Conditions for Consent"), require that data subjects have consented to processing their personal data. One of the ways to do this is by adding a consent question to the beginning of your survey.
Often privacy laws or internal organization security rules require that personal data not be retained longer than necessary. By setting data retention rules you can specify after how much time contacts and/or respondent data must be removed automatically from your surveys.
According to article 15 of GDPR data subjects have the "Right of Access". If a contact asks for the data you processed, this article will show you how to lookup a contact across all your surveys and how to review and download that information.