The first thing to say is that the Sony Reader range of eBook Readers don’t need to have external power supplies, as they will charge up via the USB cable when connected to your computer, but they are useful accessories for a number of reasons.
The main thing is convenience, as the big difference is that when using an external power supply, the Reader is still able to be used as a Reader – it’s not going to just display the “USB connected” screen that you get when it’s plugged into your computer!
Another advantage is that the Readers charge significantly faster via the external supplies than they do via USB, so you’re less likely to have to choose between missing the bus or having nothing to read! This is particularly applicable if you’ve let your Reader’s battery drain completely, as from flat, a USB-connected reader can take a very long time to even get enough power to switch on the LED to show that it is charging!
Finally, the external power supplies are much more convenient, being able to be plugged in anywhere you have a mains power supply, while the USB cable needs to be plugged into your computer.
Actually, that bears emphasising…
Wall chargers using USB connectors don’t work with Sony Readers
The type of charger you need is one with a round connector that will plug into the round (yellow) socket on the bottom of your reader. Note: While the official Sony item is quite pricey, a ‘generic’ Sony PSP charger will do fine (as long as it’s the type with a round barrel) and be much more affordable!