The dual-voltage shaver outlets have certainly been around a long time. I can only assume that someone in the past decided that an electric shaver is the sort of thing a tourist would be likely to bring along and that it would therefore be a good idea to provide 110-120V.
Once one manufacturer did, the others probably followed suit and the idea just stuck. Don't forget that in the past some Continental visitors might also have had shavers designed for the 127V supplies that were used in parts of France, Spain, etc.
As Hutch mentioned, a single receptacle with a switch to select the voltage was commonly used. I think there are still one or two models which use this approach.
There are also types where the receptacle is designed with three connection points in a row. You just plug into whichever side you need for 120 or 240V.
