You aren't missing anything.

Sellers are often wrong, remembering only the year in which they bought the bike, which of course is not the same of a model year.

I do care a little bit about year when I buy used frames sight unseen on eBay, because frames from the late 1990s and early 2000s often are built around a shorter top-tube and longer stem geometry that I do not like. Given a year, I can sometimes find the geometry on the manufacturer's web site.

In mountain-biking you sometimes have yearly changes in suspension design and shock technology that matter, and the model year can be a shortcut to discerning just what it is that you are dealing with. For example, I recently learned of a change in the 2013 year on RockShox brand forks that makes shortening the travel a more difficult and more expensive task than in prior year models.

I used to know the year of the frame I'm currently riding, but can't bring it to mind right now. 2002 or 2003, I think. Parts are all a hodgepodge of newer parts that were never spec'd on the bike to begin with, so I can't really assign a year to the bike as a whole, but only to the frame.