A programmable FPGA SoM in the tiny microSD form factor

I first I thought this was a regular storage microSD with an FPGA that allows you change the data live as as it is saved or something. But seems to be an fpga that has microSD connection with no real storage capability like you would have in a reguar microSD (other than storage for fpga bitstream), i.e it is not storage device use case. But why microSD? Is it just because you can load the bitstream without having to use uart or jtag?

"The Signaloid C0-microSD has two main use cases: You can either (1) use it as a hot-pluggable FPGA module, or (2) use it as a hot-pluggable Signaloid C0 RISC-V co-processor module."

That is not really a use case. Use case usually gives examples of how they are used in production, i.e, more specific about applications.

The SD Card interface spec includes general-purpose IO in the form of https://en.wikipedia.org/wiki/SD_card#SDIO_cards , which has been used for plenty of things that aren't storage. As to why you'd use it here - I assume the appeal is that if you have a SD slot on your computer (which is quite common) then you can plug this in and use it with no additional hardware at all.

If I buy it and plug into my computer, I guess my computer will see it as a storage device. If I send it a picture, does it overwrite the fpga bitstream? Do I have write custom drivers so that it does something useful?

I would not expect it to show up as a storage device, no; the point of SDIO is that the SD slot is generic like a USB port. You would need software that can use the device you've plugged in just like if you plugged in a separate FPGA programmer.