Depending on your site, you’ll also want to include who may have access to the information you collect from visitors. Do you share data with a third party? Does anyone in your company have access to the information? Ask yourself these questions and consider including the answers in your policy.
One of the most important reasons to have one is of course legality. Without one you increase your risk of being sued. This can happen to big players, like it did with Google and it can happen to small sites and businesses as well.
The public cares about their privacy and personal data. You have a privacy statement shows that you do too. The more clear and thorough you are with your information the more your visitors will feel comfortable with your site.
However you can also go through the Better Business Bureau or other organizations to get good information if you’d like to build your own.