That is an excellent question:
https://en.opensuse.org/SDB:System_upgrade
Among the advantages are:
You only download the packages that need to be upgraded, thus using
a lot less bandwidth.
During the upgrade, you can still use your workstation (even if
this is not recommended); the only downtime will be the reboot after the
upgrade.
You do not have to use a DVD, nor do you need a DVD writer. (You
also could boot from the net or a USB key, and install the rest from the
net...)
The disadvantages:
If, for any reason, the upgrade is interrupted (e.g. power outages,
network disconnect) and the process cannot continue, you could be left
with a broken system (that depends on where the process stopped of course).
If you have multiple systems to upgrade, you use bandwidth each
time, so it might be better to download an ISO image.
It does not do all of the cleanup and maintenance that an offline
DVD Upgrade does.
-Regards