A basket can look complete while still leaving the customer unable to use what they have bought. A camera body without the right mount, a replacement part without its fitting, or a kit missing its essential companion creates a problem that appears after payment, when correcting it is slower for the shopper and the store. A WooCommerce required product rule lets you recognise that dependency while the customer can still act on it.
The important choice is not whether you can display another message. It is whether the relationship is strong enough to warrant intervention, what the customer needs to understand, and whether a warning is sufficient. This guide sets out a practical way to make that decision, configure a rule precisely, and test it without accidentally blocking sensible orders.