... but the airlines have it on because it is considered a good operational procedure.
It MAY be standard practice in some parts of the world (USA etc), but to be honest, in daylight, if you can't see something with a 30 metre (100ft) wingspan in good daylight conditions, it is highly unlikely that you wil1 see a 20 Watt Nav light of size 2.5cm/1" at a similar distance!
Common sense dictates that I switch on the Nav lights if, at some stage during the flight, I will need them (saves forgetting later on). If it is a daylight flight, I leave them off, switching them on at destination if the next crew or daily check needs them on.
What I don't understand os people flying around with the weather radar off routinely....