A question regarding DFS

[LarryChen]

As we know, DFS(Depth First Search) is the pre-order traverse of a binary tree. Here is the typical implementaion of DFS,

void treeDFS(mynode *root)
{
    if(root == 0)
      return;

    printf("[%d] ", root->value);
    root->visited = 1;
    
    if (root->left)
    {
      if(root->left->visited==0)
      {
        treeDFS(root->left);
      }
    }

    if (root->right)
    {
      if(root->right->visited==0)
      {
        treeDFS(root->right);
      }
    }
}

My question is that why'd we need to mark each visited vertex? If I follow the pre-order , I don't need to mark each visited vertex. Any guru here can give me an explaination why?