Most computer assemblers have no idea what functions a power supply performs. So many manufacturers 'forget' to install those required functions. Then enlarge that supply. Naive computer assembers even believe more watts mean better quality.I have seen several cases where the power supply took out the motherboard and at times other components in the real world.
An example. Most all computers consume less than 200 watts continuous. And never more than 300 or 350. Selecting a supply means numbers unique for each rail. But that is too complex. So we say that computer (see above numbers) needs a 600 or 700 watt supply. Then help lines are not clogged teaching basic electrical concepts to computer techs and computer assemblers.
How many supplies are missing those required functions that avert motherboard or disk damage? A power supply manufacturer need not meet any noise, reliablity, or safety standards. Because the prime contractor (a computer assembler) is responsible for meeting those requirements.
Take an AC radio tuned to a distant station. Then move it adjacent to a computer. If a computer interferes with radio reception, then that computer (or supply) is missing essential functions. This problem is particuarly common with custom built machines because so many assemblers do not even know what those functions are let alone demand them. Because most only purchase on two irrelevant numbers - dollars and watts.