when you upgrade an OS, do you clean install or upgrade?

clean install: you make a backup, nuke the computer, install a fresh upgraded copy of the distro you want from a live usb, copy your data again to the computer.

upgrade: you wait 'till the distro' developers release an upgrade you can directly install from your soon to be old distro, you use a command like sudo do-release-upgrade

and why do you upgrade like that?