Safety and warranties are the reasons that tires have date codes. Depending on storage conditions, the materials in tires can degrade over time, even without use, and producers typically warranty their tires for five years from the date of manufacture.
The rubber deteriorates over time and through exposure to heat and ultraviolet light. This causes dry rot which makes it brittle and undermines its structural integrity, making the tire prone to failure. Knowing the manufacture date of tires helps in determining if usage is safe after a period of storage.
Tire stores typically warranty their products for four years from date of sale. However, if the owner does not have the receipt, then the manufacturer's warranty may apply from the date of production, although this can reduce the coverage time.