Embedded Architecture
When I designed a very precise (0.05625 degree precision) 2 axis tilt sensor using an electrolytic sensor (MEMS had long term drift issues I could not live with) I used a dedicated microcontroller for the sensor due to the very precise timing requirements.

The application processor was a STM32F series part, but had all sorts of interesting interrupts and processing going on and therefore could not guarantee the timing precision I needed.

Apart from that, the internal ADC was never going to cut it (I needed a minimum of 16 bit precision (true 16 bits ENOB, not something usually available in microcontrollers) so I used a dedicated (and somewhat expensive) ADC. The support circuitry was also high precision and I needed a microcontroller as I was doing 64 point decimating FIRs on the averaged data from the ADC.

There were interrupts going on, but the specifics of the architecture guaranteed they could never overlap (nested interrupts were impossible) and the timing of events was guaranteed.

The results were passed up to the application processor using DMA.

This was driven by the requirements of the design. If there are tight requirements, it is perfectly possible to need to have a dedicated chip to handle some of it.