It's actually like wearing long-underwear all the time. It's their holy garment. The garment is worn at all times (with the exceptions Kagom mentioned) by members to constantly remind them of their promises they made to God.
I'm in no way defending mormons at all, I hate mormons with a passion, but I hate ignorance as well. Mormons are fucked up, but they aren't all fucked up to the point of doing shit like poligamy and digging people up to baptize them(which is just fucking wrong.)