The way I think of it, is all my equipment has a limited lifespan, no matter how well I take care of it. The capacitors in my monitor will eventually need replacing, my power supplies and mother boards will fail, and even the fans will wear out.
Every hour that is spent being on brings my equipment that much closer to failure.
For some things this is not so much an issue, as they are cheap or easy to replace, or they will last almost indefinitely. But for those things that I mentioned, and I regard PC power supplies with an especially critical eye, this is a very real concern.
I bought several Dell Dimensions years ago. I distributed them among my friends and family. All of them have failed except the one that I own. I believe this was because I turned my computer off at night and during times when I knew I wouldn't use it, like when I left the house. I think that this conserved the lifespan of the computer. The rest of my family left them on basically 24/7. Maybe I'm being overly careful, but I really do love my monitor in my cab, and it would be very tough to replace in the event of failure, so I turn it off when it's not in use. That includes tripping the switch on the surge protector to off, so I suppose I also unplug my whole setup as well.