Inexpensive, safe and fast way to transfer data to any computer from the memory cards without moving the camera and the wire for transfer. Also as some others have already mentioned, you can use the camera meanwhile second card downloads info to a computer.
For example: in my house we have a designated computer with the entertainment center which luckily has a built in card reader, but my computer, my wife's and my daughter do not have card readers, so to save time I can give the flash card reader and memory card (locked) to my daughter and let her pick whatever pictures she wants without having to worry about the camera.
I just picked up a pocket USB Flash reader for 5.99$ which is way cheaper then a D90.