TLDR: to look for resources, and for science. Or for the flex. Maybe a little bit of everything.

In the end it seems like they're not 100% sure what they'll get out of it, which is kind of cool in a way.

To make America mad so that we will try to dig a deeper hole first and end up accidentally drilling right into hell.

The Great Hole of China.

presumably it's not seismically active there, but it would be interesting to see what happened if a deep bore hole hit magma (DIY volcano)

Digging to the center of the earth(or at least the mantle) has been a huge potential flex for over a century now

To see if they can drill any deeper than your mom.

Not digging to China obviously.

I wonder if they're trying to get a leg up on natural resources to corner more markets. They're already an economic powerhouse when it comes to rare earth elements. They also have a pretty strong presence in the natural gas markets (by way of Hong Kong). I don't know how they're set for oil or mainland natural gas, though.