An example: Django backed by Postgres. I tend to view this as 1 architectural unit, i.e. Postgres is wholly embedded in Django. I am under no illusion that I have both a Django project and a PostgreSQL instance. I have a Django-backed-by-Postgres. I can have that PostgreSQL instance be a standalone interface, but that means increasing my architectural units from 1 to 2. Instead, if I want to integrate with Django's raw tables, I'm going to do it on Django's terms (via a custom HTTP API) rather than fighting the ORM over who gets to DBA the database. Bad for performance? No doubt. We'll worry about that when we get there.
Yes, you can run a web app server directly out of Postgres without an additional "app layer" like Django (Crunchy has some cool tools for this). But should you?
To be clear, I'm a big fan of KISS, just skeptical of false minimalism.