Any example of a necessary nullable foreign key?

Question

Customers
 customer_id

Orders
 order_id
 customer_id fk

If I have two tables and define a foreign key on customer_id in the Orders table, by allow

Answer 1

We have lots of these things as our application is something that starts with some basic information for an event and over time, as the event is more fully planned, more information is added. But when the information is added we want to make sure it meets the FK constraint. FKs are for data integrity but all data is not always known at the time the initial data is inserted, so nulls are allowed.