A Return & Refund Policy isn't required by law (unlike a Privacy Policy), but you're required to inform your store customers about your policies on returns and refunds if you wish to enforce them. For example, in certain U.S. states, you need a Return & Refund Policy if your policy is different from the rights granted by that state.
A Return & Refund Policy is useful regardless of the type of ecommerce store you have (i.e. website, app, marketplace) and regardless of the provider you're using (i.e. self-hosted store, WooCommerce, Wix and so on).