The auto manufacturers created the idea of an annual car model to help sell cars. Microsoft did that with Windows 98, ME and 2000. It reminds you that your product is getting older and time to replace. I imagine that many people decide to replace their car when it is "X" number of years old, instead of after "X" miles.
IMHO, it is an extraordinarily successful marketing tactic (although I don't have any evidence to prove it).