If the motherboard was knackered you're entirely correct a backup and a boot disk would not help. Until of course you got a replacement motherboard and then you could use them to get your PC back to what it was before.
Boot disks can be handy if something happens and you cannot get windows to start. If this happens then how would you be able to get to the info on your hard drive backup? That is when a bootdisk is useful. You can boot from the CDROM bootdisk and reinstall from your Acronis image.