Many reasons for different voltages in a logic circuit. But I confused as to why you don't know _your_ reason? What is the application of this logic circuit (i.e. how will it be used)?
And when you say "CMOS transistors" do you mean "CMOS logic gates" as in the 4000 series or the 74HC00 series, etc.?
bq). Even though the input goes to 24 volts the chip itself will limit is to the rail voltage of 12 volts.
You might need to include a series current limiting resistor, or the protection diode in the CMOS IC might blow [depends on what you are connecting to that input]. BUT, if high speed switching is required, inclusion of a series resistor may be a problem due the input parasitic capacitance being charged through the series resistor (could slow it down, and even cause current spikes in the VDD line).
