Just one reason

// Here's just one reason it's bad practice to set a pointer to
// NULL when you've freed it unless you have a good, specific reason
// to do so. There are *many* more.

bool doFooStuff()
{
    Foo* j = AllocFoo();
    if (j != NULL)
    {
        // do some stuff
        FreeFoo(&j);
    }
    // do some more stuff
    return (j != NULL)
}