Andrew Mawson was thinking very hard :
They always used to rotate phases down a street, so (say) phase 1 - house 1,
phase 2 - house 2, phase 3 - house 3, then phase 1 - street lighting,
phase 2 - house 4 and so on down the road to balance the load between
phases.

So as to as best they can, balance the load on all three phases, so as
little current as possible appears on the neutral line.