"Every good work of software starts by scratching a developer's personal itch."
(Necessity is the mother of invention)
ESR recommends looking for previous effort and building from there instead of
re-writing your own system from scratch.
Raymond indeed found several tools that achieved part of what he wished to
do (fetch E-mail messages using the POP protocol) and eventually settled on
contributing to an existing one titled popclient written by Carl Harris.
Harris noted he lost interest in the project and kindly passed the
maintainer's baton to Raymond. And so Eric started working on fetchpop.