For FPGAs in general: When latency is important. Even a $1 MCU has a huge amount of compute power nowadays, and they can simulate many things, but if you need to deterministically react within 1 microsecond to what happens on a pin, and your MCU doesn't already have a hardware peripheral that does exactly what you want, you're probably going to need an FPGA.
Some recent advances, like the Raspberry Pi Pico's PIO units, have made it possible to do things that MCU's couldn't do before, like emulating a ROM chip. But there are limits to what the PIOs can do.
If it comes to pure computation and not latency, you need some very specific use case before an FPGA becomes better than a regular CPU. Most likely when it involves massive parallelism and custom caches and interconnects. Although nowadays NPUs are starting to fill that niche.
For tiny boards in particular: getting experience and having fun.
Some recent advances, like the Raspberry Pi Pico's PIO units, have made it possible to do things that MCU's couldn't do before, like emulating a ROM chip. But there are limits to what the PIOs can do.
If it comes to pure computation and not latency, you need some very specific use case before an FPGA becomes better than a regular CPU. Most likely when it involves massive parallelism and custom caches and interconnects. Although nowadays NPUs are starting to fill that niche.
For tiny boards in particular: getting experience and having fun.