While it is possible to flash most available ROCK64 images to eMMC without additional hardware, the eMMC adaptor really does make your life easier flashing images on the ROCK64. This is especially true if you are a novice  ... or you simply can't be bothered to jump though the hoops of setting the jumper and removing it at the right time, etc,. 

It just makes things simpler and more convenient, and being priced at $5 there really is no reason not to get it if you're getting an eMMC module. 

You can buy the USB -> eMMC adaptor here.