Its basically the same as why do dudes wear boxers, they offer no support, and only become bunched up and tangled.
Most Helpful Girl
Its not there to support its there to protect it, bacteria can easily get in the vagina and therefor having a layer between can prevent that usually, also as we enter puberty, our vagina naturally cleans itself so you may end up with discharge or while ovulating we do as well. Its better to get it on your underwear than on your pants.
Most Helpful Guy
Sanitary reasons and it's uncomfortable to wear jeans without any underwear since it basically rubs your genitals