Why spend money on a costly basic computing mobile phone or a tablet pc when i could buy a high performance computer ?

I think Mobility is the main benefit that a user get with smart phones / tablets over computers.

You have answered the question 'basic computing mobile phone or tablet pc'.
Mobiles are not primarily designed for computing but for communicating.
Tablets for convenience not sophistication, though the technology on these is being reviewed and developed and improved at a rapid rate.
Depends on what you will use the computer for.
If you intend to do a lot of word-processing or specialist work, then a computer may be the best option.
You may be able to download the work to smartphone too.

People can't carry computers including laptop easily anywhere therefore smart devices are getting more popular.
Now online services including email, mobile banking and other online services are making their interfaces compatible for smart devices.
The future is for smart devices like: smart phone and tablets.
Thanks, Raheel

The globalization is talking loudly. People have becomes laziness and looking for easiest way to save time. Ofcourse according to our present the time running rapidly