What is the real-world purpose of an optional 1:1 relationship?

Question

I know that one-to-one relationships can be used to split data into multiple tables for performance or security and that it is used to create a is-a-relationsship.

But aside from that things, what is the real-world prupose of an optional one-to-one relationship, expecially one where both sites are optional?

Answer 1

It allows records to exist on either side of the relationship independently, with ties between records that happen to be related. If you have a real life question about this you should add the details.