Why have a card reader? Firstly, ease of use – you an leave it plugged into your computer and slot your memory card in when you need to. Secondly, the transfer speed is around ten times faster than using your camera and USB cable. The third, and perhaps most important reason, is that by using a card reader you aren’t running down your camera batteries during transfer. This is especially beneficial when transferring large numbers of photos where you risk crashing your transfer if the battery is depleted. Card readers also allow you to save data to the card, which means you can carry them on your memory card and easily transfer to another computer.
If you have lost your camera USB cable, card readers are a cheaper and more readily available alternative, and bring with them all the benefits listed above.