"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.