This principle is actually “the first lesson” in Raymond’s book: “Every good work of software starts by scratching a developer’s personal itch.”
Later Raymond observes:
“It is truly written: the best hacks start out as personal solutions to the author’s everyday problems, and spread because the problem turns out to be typical for a large class of users. This takes us back to the matter of rule 1, restated in a perhaps more useful way:
To solve an interesting problem, start by finding a problem that is interesting to you.”
From the chapter The Social Context of Open Source Software
Because contributions to open source projects are usually volunteer work, contributors are usually driven by some personal reason to work on a bug or feature. This usually results in a developer (or other kind of volunteer) who approaches a problem with strong motivation and a passion for solving it, plus some personal experience informing their direction and creating empathy for the end users of the feature/program.
This principle can turn into the world’s most effective recruiting tool, if the project avoids perfection and communicates transparently enough. Drawing attention to where the program needs improvements, and showing the impact that those improvements might have, can make recruiting contributors even easier.
Simply put: They can’t help fix it unless they know it’s broken.