There are a lot of reasons to document your code/api/project, but one is particularly interesting to me. It’s that good documentation is a signal.
Poor documentation is pervasive, but that problem is your opportunity. Writing solid documentation can make your project stand out in a sea of alternatives. And it’s not just eye candy. It’s evidence of your commitment to the project and your thoroughness as a maintainer. It’s a sign that your project is designed for general use. You’ll usually find good documentation for mature, dependable projects. That’s a good crowd to be associated with if you want your project to be used by others.