Its merely a way to check that the same people don't answer the survey more than once, and also to give an address to email if you select the option to help with more surveys from the same site in the future (they won't contact you if you select the 'no thanks' option). BUT instead of using your email address you can type a random name in the box and it will work just fine, as long as someone else hasn't used the same name previously, in which case just make up another one!

By the way, thanks to everyone who has taken the survey so far, your answers are like gold dust to me so it really is appreciated!!