None. The whole source of wp_create_user()
is:
function wp_create_user($username, $password, $email="") {
$user_login = esc_sql( $username );
$user_email = esc_sql( $email );
$user_pass = $password;
$userdata = compact('user_login', 'user_email', 'user_pass');
return wp_insert_user($userdata);
}
It just calls insert version almost immediately, basically a shorthand wrapper. As for why it exists – core works in mysterious ways and (short of hunting down developer by version control history and asking) there is rarely way to tell. 🙂