It is exactly what it says on the tin - a flash gang writer ;-) It writes carts in a gang (i.e. several at once) by flashing the EEPROM. I have one of these, somewhere! I had totally forgotten - thanks for reminding me! heh.
It's good for dumping N64 carts, you can also plug in the tall flash dev carts to dump/rewrite them. Probably not a bad a idea to dump the contents of flash carts every half-decade and reflash the cart to keep the data from bit rot. A really nice thing to have, especially if you have any n64 devcarts. Me, I'll have to settle for a serial-driven PIC mcu for now