Do I have to wear underwear?

I just met this girl and we’ve become friends. She gets really bad UTIs and doesn’t really wear underwear anymore. And I was thinking well why do I either? It’s so expensive and during my period I wear tampons so it’s not like I need it for pads. And your supposed to let your vagina/kitty breathe at night so fuck underwear! Why do I need to spend my hard earned money on something I don’t need?