By default, there is an E-mail field provided on the Comment Card which allows respondents to provide their e-mail address should they want to do so.
However, you may have instead chosen to deactivate this question in favor of providing your contact information on the Thank You page of the Comment Card, such as the e-mail address or phone number to your company's Customer Service department.
We view the optional E-mail field as an option for the respondent to provide their contact information in case they may want your company to reach out to them. As well, when analyzing your data in our portal, you will be able to associate your responses with these e-mail addresses should you ever want to reach out to certain respondents about their experience.
On the other hand, providing your contact information on the Thank You page would provide your respondents with an alternate way to contact your company of their accord should they ever need to.
Thus, employing both methods would provide your company with 2 different methods to follow-up on a respondent's experience (either through incoming or outgoing contact), while the same could also be shared by the respondents themselves. This can become very useful if, for example, one of your respondents noticed a specific technical issue on your website which may need to be addressed immediately.
Overall, while there is value to be received in these options individually, we find that combining both methods could be the most beneficial option for you to consider.