2Y14 starts from Haymarket at 1739 on its way to North Berwick. It's formed of an EMU from Waverley, ex-North Berwick which reverses just west of the station. With wires to the loops at Dalmeny, you could use it as a turnback point for ECML suburban services in the peak. There might not be a big market for calls at South Gyle/Edinburgh Gateway, but being able to reverse in a loop might make it easier to timetable. There aren't any similar turnback points on the other routes west of Edinburgh so reinstating the capability for a heavily EMU suburban network would be useful.
Also, in the unlikely event a BEMU doesn't want to go back onto battery power, having somewhere wired up for it to stay out of trouble seems like a good idea. If the plan is to do through running from the Borders Railway, you'd want somewhere either side of Waverley to turn trains back. To the south-east it's easy as you have Millerhill but these loops are your best bet to the north west.