PIN is a four digit number.Which google adsense sends you when your account crosses 50$.This is just a verification of your address.ITs a letter containing that four digit number and you are supposed to enter that number in to your adsense account, other wiase your payments will be onhold. The profit of this number is that it removes onhold option from your payments.And as your account reaches 100$ adsense sends you the check, otherwise you can't get your payments without PIN. SOmetimes some people didn't recieve their PIN numbers and ask adsese to send it again.But still if you didn't receive your PIN then you can ask adsense to to verify your address by a phone call and adsense will make a phone call to you and your address will be verified. Hope it helps.
It's still needed to confirm that you actually exist and not using a fake address and such. It's just to keep out the non legit users basically.