Software designers prefer using IPO (Input-Process-Output) charts because they provide a simple and clear way to visualize the flow of a program. IPO charts help in identifying the data needed for the program (input), the operations or processes that must be performed on that data (process), and the result that is produced (output). This structured approach aids in organizing the program’s logic and ensuring that all necessary components are addressed. It also helps in debugging and optimizing code, making it a useful tool in software development.