The author comes at it from the open source software angle. Open is not just the license, it's a way of working, and documenting that work is vital. Documenting the process lets the world know what doesn't work and what pitfalls to avoid. Documenting the process lets others know why things were done the way they were done - what went into the decisions that were made along the way. Documenting the process lets others know if it's a project worth joining, and who's doing interesting work.
There are parallels with open education too. If we engage in open practice, others can build on our work. Maybe they will come up with something we can take back to our courses. When we did The Internet Course, we class-sourced and CRAAP-tested the readings. Bryan Jackson took our idea and built on it, and had the class work together to evaluate the readings.