In my experience, the usefulness of documentation in code declines as familiarity with a codebase increases. The result: people ignore it; it becomes outdated; now it's debt. Similarly, non-intralinear documentation (documentation that isn't in the code) tends to grow with a codebase. Meanwhile, the codebase changes, personnel change, and more and more of the documentation beco.ed noise, a historical artifact of solving problems that either no longer exist or can no longer be solved the same way.

That being said, good documentation is worth its weight in gold and supports the overall health and quality of a codebase/project. Open-source projects that succeed often seem to have unusually strong, disciplined documentation practices. Maybe that's just a by-product of engineering discipline, but I don't think it is -- at least not entirely.