Bear with me on this one. What really makes the Raspberry Pi what it is? Linux? No, because there are plenty of machines the open source OS will run on. Linux is a Unix derivative; the basis of macOS is FreeBSD, also a Unix derivative.
Sure the Pi is only 40 quid and small, but for me what really makes the Pi stand out from all those laptop and desktop computers is the fact that it makes its microcontroller’s GPIO pins readily accessible through a handy set of header pins. Unlike all ‘serious’ micros, it’s perfect for connecting to and controlling a whole stack of add-ons, including sensors, displays inputs and actuators.