In C programming, the stdio.h header file is necessary for using input and output functions such as printf() and scanf(). If this header file is not included, the compiler will not recognize these functions and will throw an error when the program tries to use them. For example, if you write a program that uses printf() to print output but forget to include stdio.h, the compiler will not know what printf is and will report an undefined function error. In short, without stdio.h, your program won’t be able to perform any standard input or output operations.