What's the relevance of date codes

Sorry, if I may ask.
What's the relevance of date codes on lenses?
Is there a database website that can tell if the batch is defective or not?

I think serial numbers would be used for dud batches, its as simple as it says, just to work out how old they are and where they were produced.

Lenses go through minor changes through their lifetimes (differenct circuitry and structural changes). It's not enough to drastically change the quality of the shots, but the codes are useful to any repair staff to know what new parts to order.

Lets say you were about to buy a used lens from someone. A lens that has been made for the last 15 years. Would you like to know when it was made? Myself, I would pay more for one that is only a year old perhaps.

Ok, now i get it.
I thought it was used as reference for tracing good or defective batches similar to the 24-105 UU1000 problem.