[ly]

On the product page (https://www.autopi.io/hardware-dongle/generation-two) on their own website they give plenty of reasons why you might want to use HDMI, a GPU, sound, GPIO and other Raspberry Pi features.

It also might have been more expensive to design and produce their own PCB with all these features, so if the Pi is cheaper, why not use it?

Also, there are a ton of information, tutorials and libraries available made for the Raspberry Pi. Now their customers can take advantage of all that too.