the stage mirrors there basically have two effects: 1, they double the persons standing on the stage, so even more virtual girls.^^ 2, they simply make the room look bigger than it is. On a third note, while standig backwards to the audience the girls can see the audience or the other girls, maybe they can coordinate their dancing better with those mirrors. but all this may be wrong. sole purpuse of the mirrors is ... the girls can steadily check their outfits and make-up during performances.
edit. oh, you ment the pillars
- there was even a feature about those pillars, Acchan introduced them quite happily XXXXD