What is the difference between passing 'the root as a copy' and 'the root as a reference' to a function?

The question is Leetcode 814. Binary Tree Pruning. Once I passed pointer to the root and then pointer to the address of root. root is also the address, that's why I tried to directly assign it to nullptr in first case. why I cannot do this? How the 1st code is different from 2nd?

1.

class Solution {
public:
   void prune(TreeNode *root){
       if(!root) return;
       prune(root->left);
       prune(root->right);
       
       
       if(root->val==0 && (!root->left && !root->right)){
           root = nullptr;  //why root is not getting assigned here?
       }
   }
   TreeNode* pruneTree(TreeNode* root) {
       prune(root);
       return root;
   }
};

class Solution {
public:
    void prune(TreeNode *&root){
        if(!root) return;
        prune(root->left);
        prune(root->right);
        
        
        if(root->val==0 && (!root->left && !root->right)){
            root = nullptr; 
        }
    }
    TreeNode* pruneTree(TreeNode* root) {
        prune(root);
        return root;
    }
};

I tried but couldn't understand, please someone help. Thank you!!