When you conduct you research you don't want anonymous users to access your survey. This will get you bad data and you will spend a lot of time cleaning up the data. Almost 80% use cases would want you to keep the survey link private and not make it publicly accessible. You don't want some spammer to start giving you feedback either. That being said there will be use cases where you don't want to authenticate the users, for instance a survey embeded on your website without any tracking information would just be accessible to anyone who visits your website.
QuestionPro offers a wide range of of authentication models at different security levels. You can chose the one that suits you the best.
This feature/tools described here are available with the following license(s) :