[nine_k]

When would a hobbyist need processing much faster than a modern beefy CPU provides, but not in a highly parallel fashion which a modern GPU provides?

My only relatively reasonable suggestion is making perfect hardware emulators for old hardware, like vintage computers and their parts.

Alas, FPGA programming model is neither easy nor intuitive, and the tools are mostly proprietary, expensive, and (as I heard) have a steep learning curve.

[vorpalhex]

These things get used in dsp and other radio applications all the time.

[outworlder]

Not just emulators. You can do nifty things, like redirect RAM reads and writes to a different location, save states, freeze the CPU, feed it with a different ROM (bypassing the one in your board), etc.

https://bitfixer.com/2020/02/19/romulator-6502-is-ready/