Your Privacy Policy agreement should inform your users about how your website or app handles their personal information. Your users must be also be informed about the reason for the collection of information, as well as how long their data will be stored on your servers.
Even if you don't collect personal information, you should disclose this fact in a Privacy Policy. It helps with transparency because users expect to see a Privacy Policy. If you don't have one at all, users may assume you're collecting a lot of personal information and not disclosing it rather than not collecting any.