The vehicles that are not sold are still in daeler inventory until they do sell. There may be special incentives or financing to make these models more attractive to buy. You can save money here, but the pro's and cons (if the new and old model have no big changes).
Pro: Save money
Con,: your brand new vehicle is a year old value wise. Unless you trade every couple of years this is not an issue. If you keep a vehicle a long time it is a total non issue.
Pro: if you are an average mile driver, upon trade in time your vehicle will have lower than average miles on it, increasing it's value.