If there's something that can be said about monitors, it is that they can never be large enough. While OS X includes Spaces which offer a way to make use of multiple desktops, there is no replacement for additional screen real estate.
Air Display enables you to use your iPad as a second monitor, extending your desktop onto it. With a screen resolution of 1074 x 768, the iPad is an effective secondary display which offers a decent amount of usable space.
When used as a second monitor, the touch screen capabilities of your iPad are still available. This means that you can interact with OS X by using your finger on the screen, but your standard mouse can also be used, moving from one screen to another.
Air Display works over a wireless network, so there is no need to connect your iPad to your computer. Whether you want to use the program to widen your desktop or provide an area that can be used to display applications you always want to be able to see - such as your email client - Air Display is a cheap and effective way of gaining a larger workspace.