When your computer shuts down improperly, as when the UPS runs out, your
software and hardware may get damaged. You may prefer to leave the
computer powered off until you can be there to assess any possible damage.
When the power comes back it may go off again shortly afterward, causing
a second improper shutdown and further increasing the risk to your system.
If you are not present when your computer restarts, and you have not
adequately password protected your computer, anyone with access to your
computer can use it.
I'm sure there are other, even better reasons to "stay off" but those
are a few I can think of off the top of my head.
On the other hand, if you had scheduled some program to run while you
were away -- a backup, say, or a virus scan -- you might not want a
brief power outtage to interfere.