>Of course types and structures are useful but why do I need it in my database as well as in my app ?

Do you write bug free code 100% of the time? I'm pretty good, but I definitely don't. And when there are bugs in my code, I'm grateful for a RDBMS with enforced referential integrity.

I've also had to work on projects where the initial codebase was scrapped all together. When that's happened I've found having the data in clearly defined columns and relationships immensely useful.