Underwear provides a layer of comfort and protection between those sensitive areas and clothing. If I don't get my nice clothes dirty and want to wear them again without dry cleaning first, I feel confident in the fact that they haven't touched my butt directly. And underwear can act as security...say you are tall and clumsy and unstable on heels, but wear them anyway because the people on tv say heels make your legs look better and as you come down the stairs in the church foyer, you crill your ankle and fall down the carpeted stairs; it is much better for the Sunday school kids to see your panties as you lay splayed and in pain than to see your goodies all out there on the church floor.