I can think of three reasons. Only the first one is really applicable at the Washington DC airport.
* Many aircraft have a pulse system in their landing lights to make them flash automatically. It is an anti-collision measure. A lot of jets around here have a system that flashes the landing lights alternately on each side. Very attention-getting. Does that match what you're seeing?
* At night if a pilot has a radio failure, or doesn't have a radio to begin with, he may flash the lights to acknowledge ATC transmissions or signals.
* Air traffic controllers who do not have radar sometimes ask a pilot to flash the landing lights, so they can be sure they have identified the correct airplane.
I'll edit this into being a tech question and ask someone on the tech side: how do pulsing light systems affect the life of the light bulbs?
[This message has been edited by Luftwaffle (edited 22 March 2001).]