How does that make it easier though? If I have the out of band knowledge that 9-5 is standard business hours, all I need to know if I'm organizing something across time zones is when our UTC offsets are.

If there are no timezones, I don't have the out of band knowledge of what standard business hours are, and I need to somehow figure this out for all parties. Abolishing timezones makes some computation tasks easier, at the expense of complicating actual human interactions.