I get it, but why are some people doing that when the testnet coins have no value.

Value is a really strange thing that is almost magical.
The time and the friction adds value even if the item is worthless.
It's not possible to control value, I don't believe. One mans trash is another mans treasure.
I was talking to a guy today who did an interview with a large exchange. They sent him a testnet for the interview, and required it to be sent back! LoL!!!! A multibillion-dollar company reeling back in their coins.
Feels a little like value.