Hard drives (for instance) are hermetically sealed. Regardless, the platters spin anywhere from 7000 to 10,000 rpm and sometimes faster. Between the platters are stylus' that are operated by a step motor. Each time the step motor moves, it's a duty cycle. When you boot, those step motors go through several thousand duty cycles then move back to the home position vs. leaving the machine running running and the stylus going through a few dozen cycles to get what it's looking for then returning to home.
On the IC side, you have the heat cycles that all electronics go through. Maintaining a fairly constant temperature and heat soak keeps everything heated evenly where constantly shutting down and reheating causes things to expand and contract (often unevenly until heat soak is reached). The changes can cause premature failure of components. Granted, these things are designed with these things in mind but it still serves to shorten the life span over time.
For a desktop cycling all the moving parts through cold start to hot start etc. causes them more wear then spinning at a constant speed. Shutting down machines for short periods of non-use is a "left-over" from the old days when large transistors and voltage regulators were used (they didn't dissipate heat well at all). Those kinds of components are not used anymore. Even voltage regulators are solid state now.