Using microcontrollers in an Introductory course
Microcontrollers can address two important aspects. 

The first is bridging the gap between the code, and the real world. Particularly if you have sensors (accelerometer, buttons) and outputs (an LED array, sound, motors). Not just to address the robotics aspect (which can rapidly get complex), but some simple game concepts (patterns, reactions). Nothing which is any harder than doing the same on a Raspberry Pi with a GPIO extension, but potentially cheaper hardware (which is important if it is likely to get fried). Also, if you want a battery powered device, or a small data-logger, a microbit is more suitable than a Raspberry-Pi.

For introductory use, I think creating a simple tilt-sensitive animation is a very effective way of making 'coding' seem more real. Yes, the coding gets more complicated if you want to do more - but starting in a block editor or python avoids this.

The more tactile, less 'black box' approach to coding seems (according to research by the BBC) seems to provide more up-front interest to less technical students.

The second aspect is working in a resource-constrained environment. Admittedly this is more a advanced concept to teach, but the microcontroller is 'clearly' a different beast to a laptop or a phone. Having some restrictions sometimes help people to solve problems more easily than giving them the blank sheet of 'plus anything you can find a python library for'. A computer small enough to wear on a lanyard will shift the way students think about what is possible.