you can use it to write custom channels into an exr.
for every channel you create under the "mattes tab" you get an input for this channel. you can name the channels and define if it is a single channel (A), 3 channel (rgb) or 4 channel (rgba) image
mainly its a technical demonstration of the new EXRIO fuses to write custom loader/saver for multi channel exr files.
with it you could create a custom exr loader which supports more then the standard fusion channels