If you use an FPGA like a CPU, its a waste of the FPGA.

This FPGA will likely be used for implementing the variety of protocols in the embedded world. Imagine that you want to attach a Raspberry Pi camera to an arbitrary Cortex-M4... and the M4 doesn't support the interface.

Instead of bit-banging the interface, you simply program the FPGA to handle that for you, and to trigger an interrupt whenever a full frame of data arrives. The spare CPU cycles are then used for sleep or maybe something else.

Tiny fpgas are sometimes used just to add a bit of glue logic next to a MCU. Or even just as reconfigurable routing for future-proofing a board. I've seen them as small as 600 logic blocks, which certainly isn't enough to do anything remotely advanced - but they're getting real cheap these days :)

Roll-your-own IO. Wouldn't work with this particular device, but for example budget oscilloscopes have an FPGA for the processor to operate the sensing system.

Implementing something you could buy off-the-shelf on an FPGA is almost always a fools' errand.

Well here is an example I wrote for a cheap Altera Cyclone board: an I2C bus monitor. It converts I2C into text and sends it out an RS232 serial port. You can then record all all the text in an emacs shell window for long term monitoring.

Small programmable logic chips in the form of FPGAs or CLPDs (Complex Programmable Logic Devices) seem to be fairly popular in lowish-volume flashcart designs for old video game consoles as well.