Why are spit balls banned in baseball...does it really do something to the ball?

A spitball gives the pitcher more control on where he wants it to go.
Saliva isn't really used, vaseline is.
It changes the weight of the ball on the side where the vaseline is put.
I'm sure back in the day real spit was used but I don't think it worked as well as vaseline.

A spitball is an illegal baseball pitch in which the ball has been altered by the application of saliva, petroleum jelly, or some other foreign substance.

Such a pitch presents an additional challenge to the hitter because it causes the ball to move atypically during its approach due to the altered wind-resistance and weight on one side of the ball.

Alternative names for the spitball are spitter, mud ball, shine ball and emery ball, although technically, an emery ball is one where the ball has been abraded in much the same way that the original cut ball had been physically cut.

Preparing a spitball is distinct from ball tampering in cricket, where the use of saliva and sweat as well as the shining of one side of the ball is not only legal but an integral part of the game. The altering of the ball's state by the use of artificial substances such as sun block, dirt or degradation by fingernails or other hard substances is illegal tampering.

Source: http://en.wikipedia.org/wiki/Spitball

Ah HA, here's a good one. Spit balls are banned to prevent the spread of HIV, which can be contracted through spit as well as sex. All it takes is a little splatter on the lips, or a spray in the eye after the ball is hit to get the infection going. Or if it's a foul ball, the person touching it may have it on their hands, and if they don't wash before eating, they're infected, no doubt. Interesting, ain't it? And no, to answer the second question, spit doesn't affect the ball in anyway other than getting it wet.