Maybe of little significance for one house, where an extra 3M doesn't make a lot of difference, but an exercise that becomes significant if you're looking at a development of, say 500 similar units.... Is that 14 or 15 miles of 15mm?
How much is an extra mile of 15mm you going to cost?
For a student, maybe it's a project designed for you to demonstrate a variety of different 'correct' answers, according to the relative costs of materials, labour etc. etc., or maybe, if we are off to theoryland, a chance to demonstrate the virtues of a one-pipe system.