What is significance of confirmation page before deleting a node?
This is no t exactly Drupal specific, but I'll answer them anyway because one could miss a few reasons. Correct me if I'm missing any. 


It prevents accidentally clicking the delete button and actually deleting it. 
It provides security against CSRF attacks. If the node is deleted as soon as the node/xyz/delete page is loaded, an attacker can make an administrator open that URL, and the node will be deleted without his intention. 


Because the specifications say so. http://www.rfc-base.org/txt/rfc-2616.txt     GET requests should not make any change in data, so the request must be either a POST or a DELETE. Since Drupal forms only support GET and POST, we use the latter.