To guarantee, in theory, that there isn't any contamination between two different flash drives.
But in practice, I'm pretty sure that with the right privileges you could just write to the SD card...
Isn't there also a physical switch on the SD card to mark it as write-protected? I'm sure that could be bypassed, but it would make it more difficult to hide the contamination.
The physical switch is entirely software implemented.
That seems quite risky tho, once the device is infected, that would be like washing all your dishes with the toilet sponge.
What is the goal of having to reboot at each cycle?