In this circuit the 12 DC, two wire is converted to three wire +5 and 0 and -5 dual supply useful for op-amps or other interface circuits. You could use 7805 and 7905 low current TO92 package.
Regulated supply is essential for precision analog circuits. This also could be used for signal conditioners that are built into the sensors. They are called voltage or current transmitters.
Negative supply also ensures total off or high-z in cmos analog switching. If you have crosstalk or leakage error in the Analog Mux; use a dual supply FET switches to turn total off and total on.
It is a 555 Chip
Datel (ge) mux chips had low on and off, but were expensive. Some cmos switches can improve with dual supply, but care to be taken that the negative voltage does not reach the ADC or uC stages.