Conversely, a date code would conceal that the Winchester 70 you got in late '65 was actually a pre-'64. Not everybody was up to speed on changes back then.
Unless there is a very obvious change in product style or equipment, it does readily cover up the actual age of a gun on the shelf so the consumer doesn't make more of it than it's really worth.
On the flip side, the early version of something - like electronics - could be a lesser product. An upgrade from 4MB to 8MB and improved user interface would leave the older products on the shelf. Retailers and marketing are real careful about that.
Seen something you want suddenly get marked down? The better version is coming. Gun store got a bargain? Whoops, Glocks got finger grips and a rail now. Too bad you got the old one.
That's the way it usually works, haven't picked up a pre-64 yet.