Why JNEXT needs an SD-card image

A real ZX Spectrum Next keeps almost nothing in the machine itself. NextZXOS, the 48K / 128K / +3 BASIC ROMs, the DivMMC and Multiface firmware — all of it lives on the SD card, and the machine loads it from there at boot. JNEXT works the same way: it boots from an SD-card image, a single file that stands in for that card.

This is why even --machine 48k needs one. The 48K BASIC ROM is a file on the card.