The original idea behind relational databases was "structure, then data": you needed to define what the data looked like before being able to insert any content. This strict data structure definition helped keeping datasets in order by verifying data types, referential integrity, and additional business conditions using dedicated constraints.
But sometimes, life can't be predicted, and data can take different shapes. To enable some sort of flexibility, modern databases like PostgreSQL® started adding semistructured column options JSON, where only a formal check on the shape of the data is done.