There are certainly different kinds of developers writing different types of tests. I usually only write the tests first if I‘m adding a critical functionality to some method or function already present. However having automated tests can help you when you can‘t easily understand the code or when you want to refactor that code to make sure you‘re not breaking existing functionality.

What you‘re describing with external devs often happens when these devs can‘t access the real data - plus you often want these tests to be automated, which usually brings with it the requirement of atomicity, i.e. you want one test run running in parallel with another not effecting each other. That usually doesn‘t work well with a real database (unless you really take your test engineering to the overengineered tier).