On all the computers (running Windows) that I've used, it only requires 10% volume to get a "loud enough" level on my headphone, but if I use the speaker I usually set it to >50%.
On my smartphone (Android), I need to set the ringtone to maximum volume, but with a headphone (or is it earphone or headset? What the...) I need to reduce to 2/3 of maximum level or I think I will be deaf.
So, with the default settings on computers and smartphones, if I just used the speaker to watch videos, and later plugged in the headphone (or whatever) but forgot to re-configure the volume level, and then you go watch videos again... you should be able to imagine what's going to happen.
This doesn't seem to be a good user experience and can possibly harm the user. Why wouldn't the operating system make the maximum audio level for headphones to "safe level", or by default have separated volume control for speaker and headphone?