As mentioned above, it's for customer convenience. Instead of having to remember a unique number or string of numbers, you just need to know the product line and roughly when you bought it.
Apple has a few ways of pinning down the specific configuration. My Late 2011 MacBook Pro model identifier is MacBookPro8,2 (which is different from MacBookPro8,1, which is the Early 2011 model, and MacBookPro8,3, which is the 17" variant). The model number itself is A1286, and the order number is MD318LL/A (order number MD322LL/A refers to the 2.4 GHz variant). So the specific numbers are there.
If you're wondering why Apple chooses to do this compared to the iOS devices, it probably has to do with tradition and upgrade cycles. iOS devices have been updated annually almost perfectly (the iPad 3 is the only exception I can think of), so there's not much need to identify it with the year. We know that we're up to the iPhone 6, so an iPhone 5S must be one year older and the 5 must be two years older. Macs are not updated annually - some lines are updated more frequently, and others less frequently. So it makes sense to give an identifier based on the year to better understand what technologies the system contains.