Visible Panty Lines. No girl wants that!
Or you can pull a 'Charlotte' from Geordie Shore and make underwear out of paper.
Some women feel its erotic and a turn on. Second best to being an exhibitionist or completely naked and exposed.
So you cant see the underwear through.. or maybe its more comfortable.. or maybe to get attention from guys if its noticeable//easier access to get with the next person they meet. :')
I don't know. Underwear is meant to protect your outer clothing. Maybe they don't care.
To save money.
This woman thinks thats slutty. Only time I dont wear it is when Im wearing PJ's