I had a discussion about conservation with a friend of mine who told me that he never shuts his computer down because it doesn't use much electricity. I asked him how he knew that because be both live in the same building and the electricity is included in the rent. He looked at me like I said something incredibly stupid.
I got my revenge when the fan on his machine failed suddenly after running 24/7 for three years. The fan failure cooked his motherboard taking the drive with it and cost him $600 to retrieve half of the files. He's a 3D artist and lost a ton of irreplaceable stuff. What goes around comes around. The moral of the story is that your expensive appliances will last long if you don't let them run while you're not using them. Would you run a bench grinder 24/7? Would you buy a bench grinder that did,t have an off button?
Leaving your computer on is a mixed bag. Turning your computer on is the biggest stress that a great many of the internals of a computer, like your motherboard, sound card, and RAM chips ever face. On the other hand, running stresses other components like fans and power supplies, and these components benefit from turning your machine off.