RS-232 drivers like the old MC1488 were run off plus and minus 12 volt supplies. DRAMS like the MK4116 also required a low current -5V bias supply. And there we

RS-232 drivers like the old MC1488 were run off plus and minus 12 volt supplies. DRAMS like the MK4116 also required a low current -5V bias supply. And there were other chips that possibly needed a -5V supply. Since the -5V was low current you could generate it off the -12V with a simple linear regulator.