In C language, the & (ampersand) symbol is used as the “address of” operator, and it allows us to obtain the memory address of a variable in our program.

We need to use the & symbol to include the address of a variable when we want to pass that variable to a function as a parameter. By including the & symbol, we pass a pointer to the memory location of the variable, rather than passing the value of the variable itself. This is particularly useful when we want to modify the value of the variable within the function.

For example, consider the following code:
void increment(int *p) {
(*p)++;
}

int main() {
int x = 5;
increment(&x);
printf("%d", x);
return 0;
}


In this code, we define a function increment that takes a pointer to an integer as a parameter. Within the function, we use the * (dereference) operator to access the value of the integer at the memory address pointed to by the pointer, and we increment that value.

In the main function, we declare an integer variable x and initialize it to 5. We then pass the address of x to the increment function using the & symbol. This allows the increment function to modify the value of x directly.

So, in summary, we need to include the & symbol in C language when we want to pass a variable to a function as a pointer to its memory location, in order to allow the function to modify the value of the variable directly.