To be fair, who actually has a mental model of what 20,779 square kilometres or 8,023 square miles (the area of Wales expressed as a number) actually looks like? Doesn't matter if you're using imperial or metric, it's not very intuitive beyond the human scale.

If you're trying to communicate 'this much rainforest disappeared' or 'look how large this landmass-sized Antarctic iceberg is' to a wide audience a well-known area is a reasonable way to do it. In Britain basically everyone knows roughly what the size of Wales is relative to the other countries of the UK, even if they don't know its exact size.