> I mean why bother having a desktop or laptop if you can use your personal device, plug it into different things and get the full experience as if you had a laptop or desktop?

Right now, tablet + laptop often replaces what I would do a couple decades ago with books (or notepads) + desktop. Sure, a single (powerful enough) CPU with all the various interface form factors being wirelessly-connected dumb systems could work instead, but then, it seems that basic CPU and storage are fairly cheap, and the interface bits -- particular the high resolution touch screens -- are the expensive bits, so there's no real good reason not to put enough brains in the individual devices to let them be standalone rather than relying on a common external brain.

With fairly ubiquitous syncing technology, you get much of the advantage of having a single "brain" device with greater flexibility and resilience against a single-device failure.