It's frequently desirable to pass a piece of data to the payment page and then have that returned along with the refCode, authCode, etc. For example, I might have my own payment or customer id in my system that I need to track. Your customized field option allows for that, but since it can't be disabled or hidden, it does not provide a safe mechanism to do so. Since you already allow customized fields, it shouldn't take much to provide the option to make it hidden.
Guest
You can send this data in the memo field and make the memo field hidden