The only way you can do that is if you're making a copy of a copy -
I guess you're really asking the question "Why do I have to backup to my HD ?"
Think of it like you're making a Peanut butter and Jelly sandwich. Everything you need is in one place, the cupboard. All your DVD files are in one place, the original DVD.
You need a place to put all your ingredients, the Bread, Peanut butter and the Jelly - so that you can assemble them into a sandwich, you use the kitchen table. You need a place to put all the IFO, BUP and VOB files you are taking from the original DVD, you need to assemble them into a VIDEO_TS folder OR you need to put them all together into an ISO image, you need a place to work, you need the HD.
You assemble your sandwich putting your Peanut butter and your Jelly into two pieces of bread. When you are finished you can one of two things. First, you can leave all the "Fixens" on then table to make another sandwich sometime later OR, you can put it all away and have your table clear again. In DVD backup, we needed the HD to lay out all the IFO, BUP and VOB files and then to assemble them into a folder called your VIDEO_TS folder OR to put them all together onto an ISO image. When you are finished you can do one of two things. First, we could leave them on the HD to make another backup DVD later OR we can have them in a temp folder to be deleted and have our HD space clear again.
So, back to our original question - If you want to do a backup of an original DVD you'll have to backup your DVD to your HD in order to work on it to make your backup disc.
You can copy a backup copy on the fly because it's already decrypted and processed to fit on a single disc.
Now that you know why, you can follow the bread crumbs back through the forest till you arrive safe and sound back in your own little bed.
