This. Developers tend to assume that users are expert in the field domain, and that they'll know everything that happens to the application when they use its controls.
But not all users are well-versed on the purpose and possibilities of the tool, so documentation should at the very least give an overall view of those. And even experts need to read about the vocabulary, peculiarities and assumptions of how the tool works in that field domain.
Some of those explanation may be taught in-app through explanatory text and step-by-step guides (which are forms of documentation), but for tools more complex than a trivial phone app, documentation explaining the tool and detailing its primary workflows and use cases is a necessity.
But not all users are well-versed on the purpose and possibilities of the tool, so documentation should at the very least give an overall view of those. And even experts need to read about the vocabulary, peculiarities and assumptions of how the tool works in that field domain.
Some of those explanation may be taught in-app through explanatory text and step-by-step guides (which are forms of documentation), but for tools more complex than a trivial phone app, documentation explaining the tool and detailing its primary workflows and use cases is a necessity.