Doubt in basic pointer

general
1

I have made a linked list, I tried to find the reason why pointer is used in creating a linked list, but later I got stuck in it.
My doubt is Why the Val and Next pointer gets changed but the Node pointer itself isn’t changed.
Also, what’s the whole point of using pointers?
I know that passing with reference will change everything, but I am not clear with the above mentioned thing.
Here is the code:

    #include<bits/stdc++.h>

    using namespace std;

    struct ListNode
    {
    	int val;
    	ListNode *next;
    	ListNode(int x) : val(x), next(NULL) {}
    };
    void change(ListNode* a) {
    	a->val = 500;
    	a->next = NULL;
    	a = NULL;
    }
    int main() {
    	ListNode* head = new ListNode(5);
    	head->next = head;
    	change(head);
    	cout << head->val << endl;
    	if (head == NULL)cout << "YESSS" << endl;
    	if (head->next == NULL)cout << "YES" << endl;
    	return 0;
    }

Output:
500
YES