drive single channel of stereo amp

I have a dumb question,

I was told long ago that it is bad to drive an amplifier channel without a load on it. I never understood why. If you unplug the source left channel from the source to preamp is that still a bad idea to play the system with everything else plugged in? Why is it bad to drive without a load anyway?
Thanks for any help.
The amplifier needs to see a load. Just put an 8 ohm resistor across the binding posts of the unused channel.
As long as there's no input, having no output won't make a difference. There are a very few exceptions to this.
Thanks for the reponses guys. What I really would like to know is why the amp needs to have a load? What causes the damage if there is no load?

Depending on the design it can become unstable especially if there is an active amp. Some designs aren't bothered at all.
My last response should have read "active input" and not "active amp".