See edit in original post. The point is that we all know when 9am is regardless of location. For those that work in office settings (which is a massive group of people), yes, 9am-5pm tends to be the rule. The reason we can do that is because 9am is more or less the same "time" everywhere.

If you don't have timezones, times cease to have any meaning. 1pm where? In New York? Great, that's after lunch. In India? Oh no, that's late at night.