History and purpose of Whomping Willow

'The Shrieking Shack was never haunted... the screams and howls the villagers used to hear were made by me. That's where all of this starts - with my becoming a werewolf. None of this could have happened if I hadn't been bitten... and if I hadn't been so foolhardy...'
'I was a very small boy when I received the bite. My parents tried everything, but in those days there was no cure. The Potion that Professor Snape has been making for me is a very recent discovery. It makes me safe, you see. As long as I take it in the week preceding the full moon, I keep my mind when I transform... I am able to curl up in my office, a harmless wolf, and wait for the moon to wane again.
'Before the Wolfsbane Potion was discovered, however, I became a fully fledged monster once a month. It seemed impossible that I would be able to come to Hogwarts. Other parents weren't likely to want their children exposed to me. But then Dumbledore became Headmaster, and he was sympathetic. He said that, as long as we took certain precautions, there was no reason I shouldn't come to school...'
'I told you, months ago, that the Whomping Willow was planted the year I came to Hogwarts. The truth is that it was planted because I had come to Hogwarts. This house - the tunnel that leads to it - they were built for my use. Once a month, I was smuggled out of the castle, into this place, to transform. The tree was placed at the tunnel mouth to stop anyone coming across me while I was dangerous.'

MOONY, WORMTAIL, PADFOOT AND PRONGS - Pg 375