If you are unsure if a website privacy policy is always required, you are not alone.
Though, at this time, there is no federal law requiring a privacy policy statement, it is very possible you must have one if:

Let’s start with the first bullet point- location

Though there is no federal that requires a website privacy policy for visitors over the age of 13, but there are state and well as international laws.

If your website is based out of or collects information from residents of one of these states, you are required to disclose your company’s privacy policy.  (click on each state to get more info on each state’s policy)

There are also international laws that you need to be aware of if you are going to be targeting international site visitors.  You can get an overview HERE.

Site tracking apps and software

If you use apps or software that collects visitor information which will allow you to contact that visitor physically or online, you need a website privacy policy.

Google Analytics Terms of Service cleary states that they require you to have a privacy policy statement on your website.

Facebook and Twitter also require that you display a privacy policy to users if you use their services to connect with those users.

eCommerce providers