My Siglent SDG-2042X signal generator uses the same SoC, would guess it's using the PRUs to drive its DACs.
A member of our local makerspace used a PRU to drive a big display made with shift-register RGB LEDs (something like a WS2812 - can't recall details).
I have made a proof-of-concept interface to a fairly high end ADC that did a little bit of filtering in the PRU before sending the results to the main CPU.
Raspberry Pi is much more popular and costs far less. The PRUs can be replaced by a couple of Arduino Nanos for a buck or two each. Won't run at 200MHZ, but there are very few control tasks that need to run that fast.
[0] http://www.righto.com/2018/01/xerox-altos-3-mbs-ethernet-bui...