Voltage is relative with GND.
"24V" indicates that it is 24V when viewed from the GND of that circuit, and there is no absolute 24V from any circuit.
In other words, 24V can exist because there is GND.
EDIT:
It should also be noted that circuit GND is a definitional.
For example, a 1.5V AA battery measures -1.5V from the negative pole in a circuit where the positive pole is 0V (GND).
On the contrary, when the negative pole is 0V (GND), 1.5V is measured at the positive pole.
0V (GND) is always required for your 24V.