Different shapes of pu are generally due to different significances. The most traditional of shapes was the Zhuan Cha or brick. This is the grandfather of shapes. They were great for stacking into baskets for easy transport.
Second, the Beeng Cha or cake tea. This shaped allowed more surface area to be exposed for more effective "maturing".
The Discus shape was a product of the Communist gov't wanting to modernize the tea producing operations. In an effort to do so, they decided that a mechanical machine could press the quanities needed as opposed to stone or traditional method pressing.
The Tuo cha is a bit more of a mystery. Some say that the shape was invented by tea maidens who wanted to make extra cash. They would pluck leaves and roll them into a ball hidden in their clothes so no one could see.
Ingot Shape, well this shape is more symbolic than for actual maturing reasons. Friends and families will give gold ingot shaped teas to bring good luck and fortune.
Jincha Mushroom shape developed from the Tuo shape. They believed that if the tea was in a shape of a mushroom as opposed to a tuocha, then more surface air would be exposed and therefore less like for the tea to deteriorate.