We could certainly survive as a species without tablets. But we have them, so we want them. As far as computing go, I think tablets are the perfect intersection between portability and function for the average consumer. Most of them don't require laptop or desktop computers, they just want to read email, play games, and check Facebook.
No one "needs" a tablet. It is simply a specific kind of computer most computers will do for the average person. A tablet is basically a design preference. It is a trend, and a lot of people don't want to be left out of trends; they want to stay on top by having the latest thing in their hands.
I believe that we want tablets, but that we do not need them, because we cannot possibly need something that we managed to live without for tens of millions of years. Most older people don't even know how to email, let alone how to use a tablet. They are a luxury.
I believe tablets are a want and not a need. I say this because there has been many, many generations in the past that did not know or even think about what a tablet was. There are plenty of people who get along fine without ever having to use a tablet.
People can easily survive life without any technology at all, so all technology should be considered a construct of man and they are categorically wants, not needs. That's not to say that they aren't useful in a variety of ways and that we shouldn't utilize them, but we shouldn't confuse want and need, they are very different things.