The 0.5(cn cp) capacitance per junction is used because it provides a good approximation of the actual depletion capacitance of a p-n junction, while also simplifying the mathematical equations used to model the device. The factor of 0.5 arises because the depletion capacitance is proportional to the square root of the doping concentration, which is the product of the electron and hole doping concentrations (cn and cp, respectively) in the p-n junction.
By using this parameter, device designers can more easily model the behavior of diodes and transistors and analyze their performance characteristics, such as their capacitance-voltage characteristics and frequency response. This can help to optimize the design of these devices and improve their performance in electronic circuits.