I guess it's just general politeness and manners, RubaDub.
Saying "I'm just going to spend a penny" is much nicer than saying "I've got turtle-neck - I'm just going to take a hot, steaming dump".
And "I just need to powder my nose" is so much nicer than "I've just hit the pad with my Devil's Waterfall - it's worse than the school prom scene from Carrie (1976) down here!"