It adds is the ability for custom configuration of input devices from within GNOME, especially for settings that are not handled by the GNOME UI tools. It's principle is quite simple. A new gsettings key hotplug-command in the org.gnome.settings-daemon.peripherals.input-devices schema points to an executable. This executable is called at gnome-settings-daemon startup, whenever a device has been added and whenever a device has been removed. Something like this:
$HOME/executable -t added -i 12 My device name
So the device with the ID 12 and the name "My device name" just got added. Other types are "removed" and "present" for those device that are already present when g-s-d starts up. It's quite simple to configure any device to your liking now. Have a look at the input-device-example.sh script shipped with gnome-settings-daemon. I'll give you two examples of what you could add to the script and the rest should be easy enough to figure out:
# Map stylus button 1 to button 8
if [ "$device" = "Wacom Intuos4 6x9 stylus" ]; then
xsetwacom set "$device" Button 1 8
fi
# map tapping to LMR instead of default LRM
if [ "$device" = "SynPS/2 Synaptics TouchPad" ]; then
xinput set-prop $id "Synaptics Tap Action" 0 0 0 0 1 2 3
fi