Answer:
In the “old days” (circa 1984), PCs used a lot of chips, and the system would tend to run hot. Each time you turned the system off, it would cool down again. Over time, the repeated expansion and contraction of the components caused small stress fractures in the soldered connections, resulting in premature system faults. The prevailing wisdom was to avoid this phenomenon by simply running the PC constantly so it would stabilize at a running temperature and avoid thermal cycling.
Today, there are far fewer components in a PC, and the vast majority of heat is produced by the CPU and GPU (graphics processing unit), so there is little concern about thermal stress problems. Instead, the issue now is about availability—people let their systems run so there is no delay by having to boot the system each time they need to use it.
Bigger concerns now are that electricity is a lot more expensive than it was 20 years ago, there are many more PCs in service, and environmental concerns have become more prevalent. PC manufacturers have worked to find a balance between availability and power consumption by developing a series of PC power-saving techniques, such as using Standby mode, Hibernation mode, and even CPU speed throttling. With these options, the PC uses less power when it is idle but is available more quickly when you want to use it. However, if you plan to be away from your PC for an extended period of time, you can certainly turn it off and conserve 100% of that energy.
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