A survey measures a person's personal thoughts, and they are designed for a variety of different people to ensure a variety of different responses. The answers shouldn't be tainted, especially with surveys that deal with controversial topics. That's why market research companies or institutions will make sure surveys remain confidential.
A person with certain opinions might opt to voice them on the survey. However, if their name became public it might cause a problem. So if you are taking a survey read the fine print carefully. Find out if your name will be withheld from the public. Also check to see if you even have to include your name and personal information.