A WooCommerce cart message by product is useful when the basket reveals something a product page cannot reliably know: the shopper has selected a particular item and may need a timely piece of guidance. That might be a fitting that needs a matching part, a product with a setup condition, or a choice that is valid but worth checking before payment. The hard part is not displaying a notice. It is deciding which product should trigger it, what the customer needs to do, and whether the message should guide or stop the order.