What is stored materials for in AIA invoicing?

When creating an AIA invoice, there is a column for Stored Materials:

Invoicing for stored materials is a way to invoice part of the contract value before work is completed. This is most commonly used so the contractor can invoice the customer for materials that need to be purchased before starting the work. Once work is underway, the value is moved from stored materials into completed work to signify that the materials have been "used" and work is underway.

In this example, we can see a job that requires $1,000 to be invoiced for materials before work is completed:

Then in the next invoice, the work is half completed, and all of the materials have been used (and therefore reduced to $0):

Information

When the project is completed, the Stored Materials column should be reduced to $0 on every line item.