K&R2 p.93 says:
  "A pointer is a variable that contains the address of a variable."

Is it true?  I think:
  "A pointer is a type that can hold the address of an object or a
   function, or can refer to an incomplete type."

K&R2 says:
  "Pointers are much used in C, partly because they are sometimes the
   only way to express a computation, and partly because they usually
   lead to more compact and efficient code than can be obtained in
   other ways."

What does "to express a computation" mean?
I came to a conclusion that "to return a set of results of a function."
Is it OK?

    void func(int a, int b, int *sum, int *diff)
    {
        *sum = a + b;
        *diff = a - b;
    }

In traditional C, a structure type value cannot be returned.
So pointers are the only way to return a set of results of a function.
But ANSI C allows a function to return a structure type.

--
T. Sakamoto