A bit like Mac - I don't bother to back up the basic operating system or any of the software that came on CD or install disks. Anything I've downloaded that I reckon I'd want to reinstall quickly is on a CD in the rack in my study.
The thing I don't want to lose is my data. That's the backup routine that goes to the USB drive.
At the moment, I use the laptop in the Norfolk hideyhole, but the next new PC here will see this one go to Norfolk, with the opportunity to back up key stuff between the two using the VPN.