What was the pit in the pyramid used for?

The pit is a chamber that has still underneath it a sealed off shaft that leads to a tomb chamber. When they place a mummy in the tomb chamber with his furniture, they seal the shaft entrance, leaving only a floor of the pit. You can see the design in the Mastabas. I am still not sure why they did not dig yet under the pit. Probably there is water and it can collapse the pyramid.