Orienteer writes
Purpose? An easy way to spot people who have crossed Z1 'illegally', I
guess.

There are plenty of ways to 'cross' London without going through zone 1 on
the tube: buses, N London line, S London trains.

Thank you for pointing out the bleeding obvious. I though even a
slow-witted person would be able to surmise from my post that as the
coloured triangles make it easier to spot a ticket's origin, it means
they can be easily highlighted for closer examination

--
Dave