The compiler and optimizer is entitled to elide certain checks or simplify code under the assumption that a pointer being dereferenced should not be null, and this could lead to dangerous things.

Here's an artificial example:

  int x = 0;
  int *p;
  if (...some condition...)
      p = &x;
  else
      p = NULL;
  print(*p);

  int x = 0;
  int *p;
  p = &x;
  print(*p);