One can neither use the same headphones for a Mac and iPhone, nor charge an iPhone from a Mac using the cables that come with the devices.
I barely use that crap adapter because it's inconvenient. Thank goodness Macs still support headphone jacks.
* To listen to the environment as well.
* To take phone calls.
* To extend the battery life of continuous listening by listening to one, charging the other, and then switch.
The latter is very much true, but my headphones (not Apple ones) pair to two Bluetooth hosts at a time. I use them regularly with my laptop and my iPhone.
You have to pause the audio output from one to use the other, granted, but it's not a big deal.
Portable Macs, sure. But USB-a charging ports are everywhere now - powerboards, wall outlets etc. If apple included a type-c to lightning cable you/someone would complain it doesn’t work with those.