Yo dawg, I heard you like MCUs, so I put an MCU on your MCU, so you can use your MCU when your using your MCU 0.o
Why?
https://www.kickstarter.com/projects/raspitv/raspio-duino-affordable-arduino-programming-on-ras
The Raspberry Pi doesn’t have a realtime unit, so it can’t do certain things, like timing loops for WS281x output. Smaller controllers execute code in realtime (ie. without a task scheduler, meaning only one program can run at a time), while embedded-Linux controllers will need a realtime unit to do that. The BeagleBone actually has two built-in PRUs (Programmable Realtime Units) which run at 200MHz, giving it the ability to bit-bang with tight precision if necessary.
This concoction allows realtime code to execute from a Raspberry Pi, which it can’t do on its own.
Raspberry Pis is not a micro controller, it’s a tiny computer with a full operating system. In principle, this product isn’t much different from having your desktop computer communicate with an Arduino via Serial USB, which is a pretty common thing to do.
Before this gets out of hand, I’m well aware of what an Atmega328 is and what the raspberryPi is.
The pointlessness of this is that its an arduino that’s a shield for a raspberry Pi. When an ordinary arduino has a perfectly good USB port, that will connect with the USB port on the raspberry Pi, and the raspberry Pi is quite capable of running the IDE.
Its a board, for a boards sake.
The meme was tongue in cheek 
Sarcasm detector failure 
But yeah, you could make it work without this board, but this achieves the joining of two pretty elegantly. No cables, just stacks on the GPIO headers, and fits super snugly on top of the Pi in a small and cohesive combined unit. The rest is just software.
I think the pitch here is primarily the very Pi friendly form factor.