Why do we do these?

spittttttt..
Krrrrrarghhhhhhhhhhh Thooooooooooo..
Pstrrrrrrrrrrrrrrrr..
Why does one do that and just about anywhere? Any time!
I just have not understood..the very fact that I just never get that feeling to spit any time, even in private (except while brushing, oh! I make noise then!) means its more a habit or involuntary act..I don’t see it serving any purpose except when people have cold. Then I wish they stay close to a wash basin and thoo to their lungs content.
Especially while traveling in buses or even while riding a bike..i just happened to see one creature dangerously riding a <100cc bike with his wife and 2 kids and maneuvering thru the morning traffic and just spitting along as he was cruising, unmindful of the folks just riding close to him. Many a times I have caught them up and told them its shameful to spit esp while riding or from a bus and all I get is a stunned stare, meaning ‘so what?’..
Its not that its just a Desi sport. NO! I have caught the likes of ponting, Akthars etc spitting in the field on live camera action! There were many whites who used to do so in Europe though this is v high in middle east and east!
Folks, pl advise any body who wud listen to you to refrain from this nonsensical habit..