>From those who say "the source code explains everything"

Because theoretically you should be able to describe not only your entire application logic, but upper and lower bounds of inputs as well. Good code would describe this inherently.

Documentation is only useful when a) the application is not source available, so you have no choice, b) you want to save a human developer time for them to understand your code, or c) you want to use documentation as a cache hit for agent use (less token spend)