From Simple Wiki

WIA stands for [Windows Image Acquisition] and is the driver standard promoted by Microsoft for basic scanning functions that don't require additional software other than the Windows OS.

WIA is much better suited for flatbed scanners than for document scanning applications. Many WIA drivers only support the scanning of a single image at a time. If your TWAIN scanner shows up with "WIA" in the driver name then you probably need to separately download a fully supported TWAIN or ISIS driver to unlock all of the features of your document scanner.