Picture the scene:
Currently stationed overseas and need to attend a meeting at Northwood at short notice. Already having plans to spend a weekend in London with friends I check the cost of changing my return flight to a Monday evening, a mighty 70 Euros, and that I can do the meeting on a Monday morning.
Check in with the requisite travel cell to tell them how clever I am and that I can save the MoD a fortune.
Oh no sir we can't do that, I'll book you a return air fare on the Monday
350 Euros later and 2 round trips in 72 hrs I got home...