With that said, though, there are also tons of inexpensive ways to output SPI or various other serial protocols from just about any device with a USB port, like

With that said, though, there are also tons of inexpensive ways to output SPI or various other serial protocols from just about any device with a USB port, like your full-sized computer: https://www.adafruit.com/product/2264 https://www.adafruit.com/product/2264 The RPis and friends just optimize the workflow - theres nothing particularly magic about they way they implement SPI or GPIO, they just have it out of the box because its such a common way to extend hobby computer boards.