Why call addlink and setlink in WCF dataservices

Question

Consider a customer has many orders.

context.AddLink(customer, "orders", order);
context.SetLink(order, "customer", customer);

The data store will be updated correctly with one or the other of these calls (after calling SaveChanges). So why bother calling both as is shown repeatedly in the documentation for WCF DataServices?

Answer 1

for adding a ref in a "to-many" relationship (new order associated with a customer) call AddLink. For the other way around ("to-one") call setlink.