postgres – Do I really need Relational Database Releases, for example PostgreSQL?

I know that triggers can be used to check stored data to keep the database consistent. Why should you not validate data on the application page before saving it to the database?

For example, we store clients and we want to do some checks that are not easy to do at the DDL level.
https://severalnines.com/blog/postgresql-triggers-and-stored-function-basics

Another example is the audit.