Lets say you have a Person table with with a SSN column. The column has a unique constraint because no two people have the same SSN. But the column in nullable,

Lets say you have a Person table with with a SSN column. The column has a unique constraint because no two people have the same SSN. But the column in nullable, since not everyone has a SSN. It would be weird to expect that only one person does not have SSN.