Airbus v Boeing
I always thought it was an automatic airbus feature. When the wheels go up the exit sign lights extinguish which may or may not be accompanied by a chime and when the landing gear is extended the exit signs illuminate accompanied by a chime. In some airlines, this may also indicate the beginning of the "no contact period" when crew are not to call the flight deck for any reason due to the high work load and critical phase of flight, ie take off and final stages of landing. There are specific ways the cabin crew would be able to alert the flight crew and other cabin crew and passengers of some type of emergency situation in this instance without the distraction of making a call to the flight deck on the inter phone. It may very well be a legacy feature of the days when smoking was allowed but I think if it has any useful feature these days that would be it.