Doubt about pointers and return of functions

Hello everyone. I would like to know why the function below would need a pointer to the pointer, in this case a **root , so that the line / 1 / could modify the *root , while the line / 2 / can modify it without having to resort to it. What is the difference between:

root->right = remover(root->right,valor);

and

root = NULL; ?

Are the two not trying to modify the pointer without using a pointer? So the two should not be wrong?

FUNCTION:

Node * remover(Node *root, int valor){
if(root == NULL){
cout<<"Elemento não encontrado."<<endl; 
}
else{
if(root->data == valor){
    root = NULL;
    cout<<"Elemento removido!"<<endl;
    return root;
}
if(root->data > valor){
    /* 1 */ //remover(root->left,valor); não funciona
    /* 2 */root->left = remover(root->left,valor); // funciona
    return root;
}
if(root->data < valor){
    /* 1 */ //remover(root->right,valor);  não funciona
    /* 2 */root->right = remover(root->right,valor); // funciona        
    return root;        
}
}
}