Delivery confirmation is very useful. I haven't 
needed to draw upon it for any cel mishaps, but 
it has come in handy. A person I was dealing with 
had moved, and accidentally gave me his old 
address--- without having filled out a "Change of 
Address" form at the post office. I was able to 
plug my Delivery Confirmation number into the 
USPS website and track that the delivery had been 
attempted, and that the package was on its way 
back to me. 

After delivery has been confirmed, the 
information on the site merely lists the time of 
shipping and the time of receipt, but while it's 
en route, it gives you an idea of how far along 
its way it is.