As a maintainer of a project, you are one of the people who knows a lot, maybe most, of the internals workings of the code. That helps when answering questions and even more when documenting the project and its workings.
Documentation is of course there to describe how your product or service works, and to cover as many details as possible so that users and developers can find answers to their problems by reading rather than asking questions that take time for other humans to respond to.
By extending and polishing the documentation, you ideally set yourself up to offload work from your future self from questions.
Providing clear and accurate documentation is hard and I have learned that you can spend a lot of time on that. What you think is a clear description might not read well in others' minds.
Documentation also needs to be easy to find (in) and it needs to be presented in a tasteful or maybe even attractive way.
I always end up spending a large amount of my time writing documentation.