What does a toner do?

It's used to balance skin pH right?





But I'm wondering.....why do we need to have a certain pH level? Does our skin become damaged at a certain pH or something? What happens if the pH is too high or too low?What does a toner do?
Yeah, it's used to balance skin pH to prevent breakouts.





For instance, since the T-zone is usually oilier than the rest of your face, after applying toner after a period of time, it's supposed to balance them out.What does a toner do?
it tones stuff.