Well the skies aren't empty, there is AIR, so particles that will reflect light. A void is just that.... a VOID...nothingness...

As for why they switch their lights on:
Pilots normally turn on all the landing and taxi lights before taxiing onto the runway for departure and turn them off again just after exiting the runway after landing. This is regardless of whether it is night or day, the purpose being to allow other aircraft to see us more easily. Although the lights do help illuminate the runway at night for takeoff and landing, it's even more important for other aircraft to be able to spot us easily.

We have taxi lights specifically for taxiing at night, and some very busy airports have a directive for these lights to be on even in the daytime, again for improved visibility.

Not all planes have logo lights to display the name of the airline, but if installed they're normally turned on at night.

Generally it's a policy to turn the landing lights off after while climbing through a specific altitude, and to turn them on again descending through a certain altitude. Again, this is true night or day, making ourselves more visible to other aircraft at relatively low altitudes (lights are on usually up to 18,000 feet).

Some aircraft (Airbus, for example) have speed restrictions with regard to the operation of the landing lights and they'll usually be turned on or off usually passing through 10,000 feet. Below 10,000 feet aircraft are restricted to flying no faster than 250 knots for air traffic control purposes, and below 10,000 feet the speed limitation for the landing lights is met. This speed restriction for the lights has to do with how they extend and retract when they are turned on or off.

Hope this sheds some lights....