I have a question which has been bothering me for a long time:

Each time I buy a ZED ticket from the staff ticket office of my airline, they stress the importance of getting listed on the flight(s) I intend to make use of, which in most cases means I have to call the airline(s) and give them some info like my name, ticket number, travel dates etc., although some airlines have the option to get listed online, which is a nice feature since it saves a lot of phone calls as some phones at exotic airline offices do not seem to be attended very often...

Now, I generally get myself listed, but I've never really understood WHY it is such a good thing to do this?
What exactly is it I gain from getting listed?
If I have a ZED ticket and present it at check in, shouldn't I get a seat if there's one free?
I've been told it has something to do with getting priority over other standby pax, but I plan my flights well in advance, avoid flights which show only a few open seats and always have several options available, so do I really have to bother with getting listed each time?