Because it’s very common for attributes to be optional but unique if specified e.g. your fleet management might have a 0-1:1 relationship between employees and cars, no car can be associated with 2 employees, an employee can’t have two cars, but any number of employee can be car-less, or car employee-less (lending pool, or cars which have not been attributed yet).
This also fits perfectly well with the normal interpretation / treatment of sql null as “not a value”.
This also fits perfectly well with the normal interpretation / treatment of sql null as “not a value”.