Correct me if I'm wrong, but I thought it was less stressful to keep a computer running constantly instead of turning it on and off. Something like the constant, steady voltage is less stressful than constant fluctuations.
It all depends.
When you turn the computer on and off:
1. It does trough a temperature cycle, which can result in cracked boards etc (because they expand then contract).
2. Hard drives have to spin up, which causes more strain on the motor than just spinning; On the drives that park their heads on the platters, the heads and the platters wear while the speed is not enough for the heads to be lifted away.
3. Incoming current surge (to charge the capacitors) can in time damage other components.
When you leave the computer on 24/7:
1. The temperature inside the case is higher, which can result in failure of capacitors (they live longer at lower temps) and other components.
2. Hard drives and fans are spinning and will wear out eventually.
3. A surge (if you do not have protection devices) can damage the power supply and the rest of the components.
Same thing happens with, say, light bulbs. Turning it on causes a lot of stress on the filament, leaving it on causes lower, but continuous stress. I wonder what the duty cycle (>0) for the longest life would be...