Warranties are designed to provide the consumer protection for a certain time period in the event that the purchased equipment (such as a handset or tablet) incurs a defect. Specific defects or issues are explicitly written into a Warranty as to being covered or not.
Warranties are "attached" to serialized Retail Products. When the sale of the retail product is finalized, the warranty goes into effect for the specified period of time.
See also:
Working with Retail Products - Warranties Tab
Removing a Status/Warranty Combination from a Retail Product