This is a quote regarding a statement of Donald E. Knuth I just happened upon here Eating your own dog food - Wikipedia (bold type by myself):
In 1989, Donald Knuth published a paper recounting lessons from the development of his TeX Typesetting software, in which the benefits of the approach were mentioned:
Thus, I came to the conclusion that the designer of a new system must not only be the implementor and the first large-scale user; the designer should also write the first user manual. The separation of any of these four components would have hurt TeX significantly. If I had not participated fully in all these activities, literally hundreds of improvements would never have been made, because I would never have thought of them or perceived why they were important.
— Donald E. Knuth, “The Errors Of TeX”