Some customers' devices hold data of such sensitive nature that they cannot take the risk, however small, of it falling into the wrong hands. The "Keep your Harddrive" warranty option is designed for these customers. If you handle data of a sensitive nature, for national security of commercial reasons, then get in touch to add it to your warranty.
The "Keep your Harddrive" warranty option allows you to log an RMA, remove your harddrive (without affecting your warranty) and then return the device for repair.