The phrase " bios9821.rom refers to an optional firmware file used in emulators for the series of Japanese computers, most notably with the Neko Project II Kai (NP2kai) core in
Index of /PC98/files/BIOS/PC98Ce2-model-S2D ... - Retro-Type
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Another angle could be compatibility with new hardware. If the 9821ROM supports newer CPUs or technologies like PCIe 4.0, NVMe drives, etc., that's a benefit. Users upgradiing their systems might need this.