As well as being in case of an electric train routed the wrong way, it also means that if that branch is later wired, then they don't have to mess around with the junction and close the main line.
Not always true that wires extending on to the Marlow branch, as exampled, are to save trouble if they later decide to electrify it.
In fact it can add to costs.
The first 400 yards onto the Gospel Oak Barking line were electrified when the North London line was 25kv'd years ago. All torn down a few months ago by NR's contractors & replaced with sticks etc feet from the ones that had been trashed. Why?