It said 4 gigs during the process, but the file is actually only 2.5gigs for me.
I think the back up is in case you f*ck up your system and have to format the disk. Thus losing HL2. If you try and activate it again, the CD key may not work, and therefore renders the game useless. The backup proves the game is yours, and it doesn't matter if you lose the original installation, so long as you got the backup.