Design and implement basically any digital circuit you can possibly imagine, within certain speed, thermal, and/or gate count constraints.

You can do any combinational and sequential logic you want without having to use a forest of discrete 74xx ICs. If you've ever scratched an itch with redstone in minecraft or taken a digital logic course you might find it quite rewarding to interpret a problem into a binary problem then implement an optimal solution with k maps and implication tables.

Practically speaking they're useful for hobbyists who want to push beyond the capabilities of the I/O and crunching capabilities of microcontrollers. In many cases the 32-bit micros around today are good enough, but I think it's satisfying to work with bare logic elements.

If you have a couple of years and a friend, it's possible to build an entire system, from designing the architecture through writing the apps to run on it: http://www.projectoberon.net

(they provide Verilog on this website, but Wirth himself has an HDL of —of course— his own design: https://people.inf.ethz.ch/wirth/Lola/index.html )

NB. Risc5 != RiscV

My undergrad computer architecture class we built a processor over the semester with an FPGA. So writing all of the different math circuits and a logic circuit and then combining them together.