Quora Answer : What is the theory behind the evolution of electronic equipment?
Basically they are trying to balance several virtues :
- small / light enough to be portable
- large enough screen to be useful.
Computers started with usefully large screens but have been getting smaller and more portable. Phones started small and portable, but are getting larger screens to be more useful. Everyone speculates that there might be a sweet-spot somewhere (so you can carry one device instead of two) but it's not clear exactly where, so there are a variety of bets : from laptops to netbooks to 12'' tablets to 10'' tablets to 7'' tablets to phablets to phones to watches and wearables to combinations of watch+phone or watch+laptop.
Furthermore "useful" varies between professions and over time. At one point reading and working on Microsoft Word and Excel spreadsheets would be the definition of "useful" across a wide range of people. But perhaps today more people can work with specialist apps. that are designed for smaller screens and touch. So the needs change.