Q:

What are the challenges for recognising the handwritten characters?

This 2014 article saying that a Chinese team of physicists have trained a quantum computer to recognise handwritten characters.
Why did they have to use a quantum computer to do that?
Is it just for fun and demonstration, or is it that recognising the handwritten characters is so difficult that standard (non-quantum) computers or algorithms cannot do that?
If standard computers can achieve the same thing, what are the benefits of using quantum computers to do that then over standard methods?

A:

Handwritten digit recognition is a standard benchmark in Machine Learning in the form of the MNIST dataset. For example, scikit-learn, a python package for Machine Learning uses it as a tutorial example. 
The paper you cite uses this standard task as a proof of concept, to show that their system works.

