Gives you the change to test new options on the same server.
..when making a child, not all the files were copied. I think it's because the PHP-routine got out-of-time. So with a large site, files should be copied manually :-(
When creating a child site, there is an option to copy files manually, or if you have big folders, you can select to copy them after manually, if you read the help it is described there.