We carry a slew of different types of Character LCDs in the Adafruit Store. The Experimenter's kit provides one, but if you don't have one, the 16x2 variety is used in the Experimenter's guide.
Character LCDs are super useful for displaying text, data, variable information or providing an interface to interact with. You've already used the Serial Monitor in previous learn guides. While it's useful to display data from the Metro, it needs to be tethered to the computer via USB. The character LCD lets you have a free-standing display for whatever you want to print - text, data, and even small icons (5px x 7px)