It's a safety feature especially if you park your car on the opposite lane (facing oncoming traffic) in dark unlit roads. Normally, when you are parked on the left (in the UK) facing forward (with the traffic), your rear lights have reflectors which reflects lights shown on them from the back by other cars' headlights. But if you are parked with your front facing the oncoming traffic, then in the dark there are no front reflectors to warn them. So, in this case you would switch on your parking lights (left in this case) so that the oncoming traffic (especially bicycles) can see your car in the dark.
That's the theory
