Being able to begin work whenever they want, and more importantly not work whenever they want. Friend calls and want's to meet at 8? Even if you have a rider you can go off the clock in ~15 minutes.
Presumably for a lot of people the thing they need to work around is not so much friends calling and wanting to meet as their primary jobs with irregular hours.
Or they have pre-existing appointments at certain times of the day (drop-off/pick-up kids at school or spouse at work if there's one car, etc). I wouldn't be so quick to assume what the flexible hours are often being used for, since it might be very different based on age group and location.
Work flexibility helps a lot of people in lots of different ways, which is why it's such a bit deal.