KenJohn

I suspect these are used for last minute travel emergencies, personal or work when you find out you have to get from A to B at very short notice.

Alternatively, where travel plans are very fluid and timing to get to the destination is essential.

I do recall a very US senior executive who was entitled to a private jet but preferred to flexible F instead. Booking and changing a crew for private jet flights are expensive and flexible F is cheaper where the airline pick up the cost of changes.