It's used occasionally to turn back late runners. I ended up in it a year ago when my Leven service got cancelled at Kirkcaldy.Does the loop get used much these days? There used to be an early morning weekday train that would run up ECS and use the loop to turn before heading to Glasgow (of all places) but I don't see that in the timetable.
You can turn back on the Down line too, but invariably there's something on your tail by Kirkcaldy so having the siding means a cancelled train can get shunted out the way.