At Google, Henner Zeller built a circuit pretty much like this that drove an RGB LED and was used for Test status 'orbs'. Basically with a bit of code your orb would turn red if your tests started failing in the continuous build.
I used one to add a foot 'key' to my setup when playing World of Warcraft (it was my push to talk switch) but it could have been anything, it pretended to be a USB keyboard with one key :-).
I've seen them used to pretend to be a mouse and move the mouse around (a 'jiggler') so that a machine wouldn't kick in the screen saver.
All various sorts of things you might want to do with a bit of compute.
So what could it be used for? Get a bunch of sensors and actuators that communicate with SPI/I2C/PWM. Think about things like accelerometers, gyroscopes, proximity sensors, liquid crystal displays, motor controllers, servo motors. Now plug them together and build just about any gadget you can imagine.
It's better than an Arduino if you're short on space, weight or power consumption. Things like tiny flying machines or wearable gadgets.
If you're a beginner looking for your first microcontroller platform, the Arduino will probably be easier to approach.