It was to make their faces stand out. Some early film stocks, for example, couldn't show blue eyes very well - this was an issue for Stan Laurel, until a different type of film started being used. Also, lighting was a big issue during the early silents. Lights would occasionally explode from over heating. In movies prior to WWI, especially, you'll notice that almost all characters have their faces painted very white, unless they were playing a non-white character, which often lead to racist characterizations. As the film stocks and lighting improved, there was less need for the white face, eye liner, and lipstick use.