Why was the inside of the pyramid decorated?

The pyramids were magical ressurection machines. They were supposed to have contained everything that the Pharoh was to have in the afterlife. These items could be represented in a number of ways. The physical object such as the chariots and food found in the tomb of Tutankhamun, or pictorial representations.

Also ofcourse, the Gods would have to have been told about the life and achievements of the Pharoh and his subjects, which is why there are texts and images showing daily life under the Kings reign.

The name of a person was important too. The tombs are filled with the official titles and the name of the Pharoh, because if there was no name associated with the mummy, then the Gods could not speak the Pharohs name and therefore he could not enter the afterlife. Finally in the tomb itself, there would have been the texts of the book of life. The magical incantations and adorations that would ensure that the Pharoh would pass the test of the "Feather and the heart".

This test weighed the Pharohs good deeds in the form of his heart against the weight of a feather. If the weight balanced then he was admitted to the afterlife. If not he was thrown to the beast. The book of the dead was a "cheat sheet" which the Pharoh could use to guarantee his place in the the ancient Egyption version of heaven.