I have long said the same thing. Granted, on most computers now there is a switch directly on the power supply, at the back of the machine. But sometimes there isn’t, and there was a window of time when the funky switches that are controlled through the motherboard had started, but power supplies with switches were rare or didn’t exist.
Still, there are ways around it, and holding the switch in really will usually result in a shutdown and off.
These days there is always a trickle of power to the motherboard so long as power is plugged into the power supply and the power supply is connected to the board. Thus the green LED that is common on the motherboard, lit even when the machine is off. I’m in the habit of turning the power supply switch off before I plug the cord into it to avoid any chance of a surge happening then.
The biggest problem is when the switch or motherboard circuitry start acting up. It’s common for me to hear the complaint that a machine won’t turn on. The usual solution is to turn off the power supply switch, turn on the power supply switch, then try the front button again.
Of course, here’s where I could get into the problem of users burying their computers in places, or under/around/between stuff, where it’s barely possible to reach the front switch, never mind the back switch, or the cables on the back.