Julia's answer, of course, is resourceful, smart and all the things, but the basic purpose is much simpler. They are needed because the girl's discharge occurs every day.
Someone uses them, someone does not - neither one nor the other is not fatal.
(Answer it seems simple and seems obvious, but a year ago I found out, meeting with a guy, that not all guys are aware of this aspect of the female body.)
I already wrote about this on TQ. Panty liners allow you not to worry about the cleanliness of your underwear throughout the day. They are needed:
● Before and after menstruation. Sometimes a pad "for every day" is used as a safety net on the eve of the onset of menstruation.
● After - to protect against residual, smearing discharge.
● During menstruation, if the girl uses tampons. The daily routine will serve as additional protection for the underwear.
● Some panty liners also have a scent that is designed to mask the smell of discharge.
● During ovulation, when intravaginal secretion increases dramatically.
● Panty liners help keep you clean when you're on a long journey and don't have the time to shower or change your underwear.