NULL exists because the following general conditions apply:
Existence –The attribute does not exist in the domain, or domain understanding is wrong. This means there is a missing entity in our domain model or entites are mixed in a table. E.g table contains hair colour for a car entity, Number of pregnancies for male patients.
Missing – The information has not been given at the time a row was created. E.g. A customer may decline to give their age.
Not Yet – Data is contingent upon an unknown event in the future, E.g. Termination date or Date of death.
Does not apply– Is not applicable for this instance of a record. E.g. Hair colour for bald people.
Placeholders – Indicates that we know that a bit of data exists, but we don’t know what it is, in this case keeping a NULL is useful for CUBE or ROLLUP queries.
In the real world applications of data structures NULLs are often unavoidable. However, it confuses users, designers and DBA’s (generally) hate it. It complicates Reporting, ETL, Business Intelligence and Data Science initiatives. As such, users need to be aware of the design and query compromises they need to use.
I think there’s significance in what NULL represents, but it’s a concept with its fair share of complexity. Read the whole thing.