Having a well-written README forces me to clarify my ideas. Just this is already a big win. But then people can come in and discover what it is easily.
I have lost count of the number of great projects that are ignored because they just lack a README.
<3 README-driven development.
When starting any new project, start by writing the README with the motivation and high-level usage sections. This helps me clarify the goal and scope of the project upfront and makes it easy to open source.
I encourage everyone to try it.