Fiona, one might see the aircraft before seeing the nav lights but amongst the din of an apron one mightn't know that the aircraft has an APU running, or could be getting close to starting.
In previous lives, as others have attested above, NAV lights are on when the PU is to be started or ruinning. Let's others know that the ship is powered, and could start without too much notice.
Re driving with lights on. I average about 30,000km/year. Much of it country, highway driving with lights on day and night. An oncoming vehicle with lights on is much easier to identify than one without, and, in 20 years of drving thus, I wouldn't have replaced more than half a dozen headlights/globes.