Comfort. My boobs don't require much support, so if a dress covers them sufficiently, I don't wear a bra. I don't wear panties with long skirts either. It's nice to allow some air citculation from time to time.
To avoid visible panty lines (VPL). I never wear any underwear beneath my pantyhose. It's really not as bad as people think - sort of like a thin panty with legs. I'm not the only one who does so. Meredith Vieira of "The View" has gone on record saying that it's redundant to wear panties and hose at the same time:
