They're not useful, they're a luxury most people in the world cannot afford, as are computers. They're MORE affordable, at least at the under $100 range, than a notebook, laptop, or tower, but still, they are a luxury the world cannot afford.
They do not raise crops, make clothing, create shelter, nor butcher meat, which are things that we NEED.
What we NEED, was useful 2000 years ago in a hunter gatherer society, which we need to return to. Anything else, including civilization, is a luxury we can ill afford and do not need.
Now if there were just some way to get rid of 99% of the world's population so we'd have enough caves to go around, I'd be happy.