A foundation footer is perhaps one of the most critical aspects of the house. It’s almost always the first building element that contacts the soil that the house rests upon.
A pier is a narrow vertical structural element made from wood, steel or concrete. The best analogy I can come up with is a table leg. Piers are often used to support a footer that’s resting on poor soil. Piers extend down through the bad soil until they hit bedrock or great soil, or deep enough to create enough friction to adequately support the building’s foundation.

The primary purpose of the footer is to spread out the weight of the structure across a larger footprint than the foundation would if it were in direct contact with the earth. 

If you were to calculate the total weight of a completed house plus everything in it, you’d probably be stunned by the total tonnage. Just the interior furnishings and possessions in an average home can weigh tens of thousands of pounds. Add this to the many tons of weight of the building materials and you end up with enormous concentrated loads.

Without a footer under a foundation wall, the wall could actually start to slice into soil much like a knife cuts into a stick of butter.