It was mostly marketing. Especially back when they started and the market was saturated with computer manufacturers all churning out their own computers that didn't interoperate well with others. It saved educators and then businesses time because they didn't have to waste time re-educating students or employees on a new system. This especially bore fruit as computers started gaining power and the ability to perform functions that had been relegated to mainframes, meaning experience with computer type X could become central to that role. I really think Apple took Moore's Law to heart and projected out the future of the role of computers in business as a result of it and the increasing shrinking of components. Why pay for a super expensive powerful mainframe when only a few people in a company might need that much power and the rest need far less? More cost effective to buy a few powerful desktops and save tens to hundreds of thousands on a mainframe.