Postcodes are used as part of the sorting process to group items of mail into specific geographical areas, as designated by the postcode for that area. By sorting geographically, the delivery process is speeded up and made more efficient. True, you don't have to have a post-code to ensure mail is delivered, but mail without postcodes can take a slower route as it has tobe sorted manually. If everyone abandonded postcode usage, the effect on the current systems would probably be catastrophic - the minority who do not use post-codes merely cause minor inconvenience within the system.