This- Even if your work allows you to set your own schedule, there many other aspects of society that don't. Schools and daycares, in particular, can dictate the schedule of parents. If you can't start off to work because the bus/dropoff isn't till 830a it doesn't matter when your office lets you come in.

Even if there aren't external factors and there is theoretically flexibility around working hours cultural norms can make it difficult. My workgroup/division seems to have a strong norm about not scheduling meetings before 930a, but scheduling from 4-5p is normal.