What does casting to void* does when passing arguments to variadic functions?

There is another question that discusses something like this: When printf is an address of a variable, why use void*?, but it only answers why shouldn't you print pointers as ints.

Another question discusses you should always cast pointers to void* when passing them to variadic functions: Argument conversion: (normal) pointer to void pointer, cast needed?. It says if you don't do so you invoke undefined behavior, but it doesn't go beyond that.

Indeed:

      if (pIReport4 == NULL)
      {
          printf("It's NULL but when I print it, it becomes: %p\n", pIReport4);
          printf("It's NULL but when I print it and cast it into (void*), it becomes: %p\n", (void*)pIReport4);
          printf("And NULL is: %p\n", NULL);
      }

Prints:

It's NULL but when I print it, it becomes: 0xc68fd0
It's NULL but when I print it and cast it into (void*), it becomes: (nil)
And NULL is: (nil)

pIReport4 is a non-void pointer.

It's clear it's pushing something else into the stack if you don't do the cast. What might it push? Why?

What's the rationale of making passing non-void pointers undefined behavior? It doesn't make sense to me...

I always thought pointer casting is just a hint to compiler how to interpret the pointed data when reading or writing it. But when passing just the pointer value I would expect that it passes the same sequence of bytes regardless of type.