[pacaro]

The best real-world example I've heard of comes from a friend who works for a travel company.

You're creating a schedule for a tour. The tour occurs some time in the future. You have a daily itinerary for the tour group.

Now the destination country changes their time zone in some fashion. You don't want to have ever stored an instant, you want the date and time to be more abstract. 9am, as it will be understood at that date and time in the future.

The same also occurs for past events, but we don't tend to encounter that too often, although the calendar for September 1752 in the UK (at the time known as the Kingdom of Great Britain) shows another wrinkle