The point of the spitball was to let the ball slip out of the fingers with less rotation than a four seamer.
That made the spitball seem to drop compared to a normal fastball.

The modern way to achieve the same effect is the split fingered fastball. The ball comes out of the pitcher's hand with less spin than a normal fastball and appears to sink.


P.s. Gaylord Perry was throwing a spitball in recent times.