The R1 is an SPU (Sensor Processing Unit) which works in concert with the M2's integrated GPU (the R1 being the innie, and the M2 being the outie). It is doing an enormous amount of image signal processing, which seems to explain the 256GB/s memory bandwidth.
https://techcrunch.com/2023/06/05/apple-r1-chip-apple-vision...: "The specialized chip was designed specifically for the challenging task of real-time sensor processing, taking the input from 12 cameras, five sensors (including a lidar sensor!) and six microphones. The company claims it can process the sensor data within 12 milliseconds — eight times faster than the blink of an eye — and says this will dramatically reduce the motion sickness plaguing many other AR/VR systems."
https://techcrunch.com/2023/06/05/apple-r1-chip-apple-vision...: "The specialized chip was designed specifically for the challenging task of real-time sensor processing, taking the input from 12 cameras, five sensors (including a lidar sensor!) and six microphones. The company claims it can process the sensor data within 12 milliseconds — eight times faster than the blink of an eye — and says this will dramatically reduce the motion sickness plaguing many other AR/VR systems."