"Undeawear" appears to be a misspelling of "underwear," which is clothing worn beneath outer clothes, typically next to the skin and under other garments. Underwear serves as a protective layer and provides support.
Long or short sleeve shirt, a nice warm comfy jacket, legging under your jeans to be warm and stylish or you don't have to wear leggings you will still be warm, comfy socks, and cute warm boots:D
There are several factors to consider, besides the fact that they are opposites. Mostly, it's about comfort. For some, it's style. For others, especially junior high and high schoolers who want to "fit in" with the other guys, it's about avoiding hazing. And, yes, for others, it's about how they look to others when parading around in their skivies. What's interesting, if you think about it, advantages and disadvantages are a matter of personal viewpoint. The advantage of brief support may be a disadvantage to one who wears boxers. And, the typical form-fittingness of a brief preferred by some is shunted by those who a re comfortable with the relatively no form of a boxer. Then, there are boxerbriefs, which blend both worlds together (depending on the brand maker of course).