Day to day, it's still much more convenient having a day definition that is roughly aligned with your waking hours, so that the change of calendar date happens

Day to day, it's still much more convenient having a day definition that is roughly aligned with your waking hours, so that the change of calendar date happens when you're asleep or otherwise don't really care about it. Solar time (even if quantized to a limited number of time zones) roughly achieves that, a single global time zone on the other hand clearly fails at that task (except for the privileged minority living near wherever the meridian ends up).