For Canada, this "registration number", or whatever you call it, is useful. When the package get out of the custom, Canada post mark it as "arrived to the country". That usually mean 4 or 5 days and it's at my door. Not fully traceable, but it's a good way to know when you will receive it.
So this number is useful for some.
A good thing I didn't asked you the tracking number for the watch I'm waiting, would be another flame war that I don't really want