According to designer Tony Guido the question at Apple was:
“What would it take to put the Mac on as many desktops as possible, without licensing, in a way that would convince DOS users to migrate toward the Mac?”
At the same time hardware engineer John Fitch, having just completed work on the IIgs, was worried by the lack of follow-up product for the Apple II. Fitch wanted to design a computer around a new chip, the Motorola 68030, which would be powerful enough for business and high end applications, but could also be offered to home users.
Inspired by the Apple II “open” architecture mindset, Fitch proposed a modular approach.
He designed a simple hardware “backbone” carrying basic operations and I/O on which the user could add a series of “book” modules, carrying hardware for running Apple II, Mac, UNIX and DOS software, plus other modules with disk drives or networking capabilities.