const is useful when declaring functions that take pointers or arrays as arguments, but do not modify the dereferenced contents:

int sum(const int *ar, int len)
{
  int s = 0, i;

  for (i = 0; i < len; i++)
    s += ar[i];
  return s;
}

int array[] = { 1, 2, 4, 5 };

int total = sum(array, 4);

The const assures the caller that the invocation will not attempt to assign to *array (or array[1], array[2], etc), even though the elements of array are modifiable in other contexts.