It's pretty obvious now that tablets are replacing PCs for a lot of people, and equally that tablets are being used by a lot more people who never used PCs or didn't want to. Despite some well documented counter-examples, most of this usage is consuming content, not creating it.
This ties directly to the form factor; the extra screen real estate of PCs is helpful, and the human input devices are much better. Tablet keyboards mostly suck, using your finger as a pointing device doesn't work as well as a mouse, etc. Over time some of these things are being mitigated, but the imblance remains.
Still, most computer usage is to consume content, and for that tablets are just as good if not better than PCs - certainly easier to use - and they are less expensive. Everyone has a story about someone they know who never used computers but now happily uses an iPad (in my case, it's my 80-year-old Mom).
There is one kind of input which is much easier with tablets than PCs - taking pictures! Which is one reason why picture-based communication like Facebook, Instagram, Snapchat, etc is growing far faster than Internet usage in general. It's easier to type something on a PC than it is to incorporate a picture, but on an iPad the opposite is true.
This creates a huge opportunity for visual search engines. As tablets continue to take share from PCs, visual search will become more and more important.