Writing documentation is an essential skill for anyone wanting to publish code for other developers to use. When a new user arrives at your project without knowing anything about it, they must be able to quickly and simply understand what the code does and how to implement it.
The documentation also needs to be able to easily express your API, the details of all the configuration options, the more advanced features of your code, and information that developers should be aware of to be able to successfully use your code.
Then there is licencing, instructions for code contributions, badges to show build and code quality information.
Often it can be as much work as creating the code itself, but is incredibly important for other users, and even for yourself!
All my open source projects have accompanying documentation: