You don't need to. However, by supplying this information, you provide the site with the means to determine your (and others') normality for specific demographics. So, for instance, if you wanted to see how "normal" you were compared to all other single, males ages 26-35, you could do that if you supplied this information. Remember, your votes will never be reveled to others, and if you choose to not share your personal information (an option in your profile), nobody will be able to see your specific demographics.