Sometimes you need to know if a value has been explicitly defined or not. Also, sometimes you need to know if a value is different from what it was before. Null

Sometimes you need to know if a value has been explicitly defined or not. Also, sometimes you need to know if a value is different from what it was before. Nullable values help in both cases, because you can tell the difference between the "default" value (false for boolean, "" for strings, 0 for numbers) and "no" value (null). If you have a form being submitted, you can tell the difference between the user setting fields to their default vs the fields not being returned to you at all. Another example is an ORM system. A data entity could be initialized from an existing database record, or it could be blank. Each property of the data entity could be initialized or not, depending on the SQL query that was used to initialize it. For fields that are non-nullable in the database, you can compare two entities for the same record to determine if fields have been updated or just weren't initialized. That lets you perform database updates only when necessary, and prevents accidentally deleting information.