What's a desktop computer?
What's a computer?
What's an external drive?
Never heard of such devices.
Backups? I know of iCloud backups, the safest form of daily backup.
Offloading? I know about iCloud Photo Library and (new in iOS11) automatic deleting apps while keeping user data.
...you get the idea
It's no longer possible to assume there's a "hub" desktop computer in the equation.
Anyway, whichever user habits are involved, having a slimmer camera roll (thanks to heif/hevc) pushes further away the moment of the inevitable offloading to a pc or a cloud.