if done properly it looks quite well. Take a look at my video...THAT is fake tan.
COUPLE REASONS....you do not want to take the chance of burning from sun burn or a tanning bed because it will cause edema. (water on top of the skin)...
SECOND: it is NEEDED due to lights washing people out. Meaning, on stage, the lights are much brighter than more room light. People who look good offstage will not show the detail needed when washed out from bright lights.
Think of this way...if you have a black car and a white car...both get dented. You will NOTICE the dent in the darker car more clearly vs the lighter car. It is just basic lighting.