Co-simulation is widely used for verifying RTL implementations, see fx. dromajo.org but it's a general useful thing and I've used in many contexts, even for verifying a compiler against an interpreter.

It's a great technique.

Using it, and a suitable test program, you can bootstrap an emulator core for something simple like a mips, z80 or 6502 in a day or two.

I highly recommend it as a low-level itch scratching programming exercise.