The reason there is a reference to equivalent focal length to a 35 mm camera is because literally millions of photographers know what a 50 mm lens will cover on a 35 mm camera as they do when a 20 mm or 300 mm lens is mentioned.
35 mm cameras were the most used in the word, in fact for may applications they still are the first choice.
Telling someone with a lot of photographic experience that you used a 400 mm lens to shoot a particular shot, they would be able to see in their minds eye a photo that would be pretty close to what it could capture. Tell them you have a 5 mm lens on a 1/2.5" sensor, they will look at you like you came from the moon. You see the sensors on P&S cameras can vary from 1/2.8" to 1/3.8" making lens lengths irrelevant. 35 mm is 35 mm.
