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.