It actually makes good sense as part of a complete backup system.
What happens to your data when your office/house/whatever with the 2 or 3 TB drives burns down with them in it, or someone breaks in and steals your desktop and the USB drive you left sitting on top of it?
Depending on the circumstances, I usually recommend RAID of some kind if possible, a USB/External Hard Drive on-site, and then some kind of off-site backup.
If your internal drive dies, if you had RAID, you just replace the dead drive. If no RAID, then you restore from your External backup. If you had a fire/theft or other major loss, you restore from web/off-site. In the case of BackBlaze, they offer 3 restoration options, included zip download of files, or FedEx thumb drive or external drive for additional cost.