Before WIA, there was little operating system support for imaging devices. Most of the components for imaging devices were provided by the manufacturer, and used the TWAIN industry standard.
With WIA, operating system components are provided, and the manufacturers only need to write a WIA microdriver to allow the device to work. This is analogous to the standard achieved in printing where manufacturers provided a minidriver, and the other print components were supplied by the operating system.
With WIA, independent hardware vendors (IHV) can concentrate more time on writing code to improve device features, rather than dedicating valuable time writing code to enable the device to interoperate with the Windows operating system.