I suppose it would be the same reason that you open your mouth to put eyeliner or mascara on........but why that is, I don't know. Out of interest, have you ever tried to put mascara and eyeliner on without opening your mouth? It is possible, but for some reason it seems to be so much easier when your jaw is resting on your chest!
It's because it stretches the face and tightens the skin, not by much but every little helps...one of those sub-concious things that people do like sticking you tongue out a little when concentrating really hard.
And louise, sorry I must be one of the few who don't do that then.....ho hum!
yes, i have tried to put on eye make up without the aid of opening my mouth, it's a bit like trying to close your mouth whilst yawning. I thought it may be because of the stretching skin thing but after trying it doesn't seem to make much of a difference. Oh well, i'll have to keep researching this very important subject. Oh, sft, you dont know what you're missing!