Your website or app must comply with all data privacy laws that apply to you, and these laws almost always require you to post a privacy policy in some way.
Data privacy legislation exists on every continent and usually has broad legal thresholds. They might apply to your business depending on factors like:
Where your business is located
Where your consumers are located
How much data you track
Some of the laws that dictate if and when you legally need a privacy policy page for your website or app include: