But what's the point then? If there's a tool, that can make "documentation" out of source code, I can just look at the source code?

Mostly it collates all your endpoints more easily, generates clients for different languages, genearates an online documentation and test page that makes your service self describing, and can be published out to third parties that dont have access to your code.

The tool parses out the bits that you're interested in. If you want to know what year the proverbial apple fell on Isaac Newton's head, you can get a biography and start reading, or you can Google and let the machine filter out the irrelevant bits.

If I'm looking at documentation, I want the inputs and outputs and a brief description of what it does. I care very little about the implementation, or I would write my own.

Presumption that all code is open source. The war is won :-)