You assume that people would be free to chose their schedule, based on their specific constraints.

However that's not how that will work out for the majority. You abolish 9-5 expectation, all fine until you get a boss that is a night owl and he wants you available from 6PM to 3AM AM. Enjoy both not having family time anymore and not being compensated to work what would be a night shift in current society.

You take the example of the lawyer here above. Right now he need to be in the office because his assistants/PA will be in the office at that time and there is no reasonable expectation he can force them to be available at other times.

This depends on what kind of job you have (and on how you are being compensated). If you made a 40 hr/wk for salary deal with your employer and now you're actually available for 40 hours while also doing your actual work outside of that time, you might want to reevaluate your work-life balance.

Lawyers and accountants can - and do - keep flexible hours when not dealing with clients. When dealing with clients, there's still expectations that the lawyer would be able to meet you somewhere within 9-5, and not at 2am. Of course, there are exceptions, but that's a convention. People want to have lives outside job, so there should be an agreement when we have "meeting points" that we're expected to be on the job, and when there's no promise.

> just a placeholder for "Oh, well those are the official times that denote when I will be available to do work stuff"

It's not "just", it's very important coordination point. If lawyers kept random hours and you needed a lawyer, it's be much harder for you to find one because you'd also look for one that has suitable hours. Not impossible, but harder. To lower transactional costs, the hours are roughly synchronized.