[almostdeadguy]

It's extremely common to validate a piece of data not on its own, but how it relates to other pieces of stored data. Without transactional semantics, an application basically can't enforce these invariants w/ any reliability (or those semantics need to be ensured out of band, or w/ little data modeling tricks that tend not to scale well).

There certainly are invariants that are non-trivial or cumbersome to enforce strictly with a schema, but you can really only enforce them w/ a database that provides serializable transactions.

In many cases, schematization of data in the database is good for other reasons though (for instance, guaranteeing type-normalized data in the presence of multiple deployed versions of an app via accident or otherwise, ensuring your queries and updates are typesafe, etc.)