Shell NZ are currently handing out promotional dockets with fuel purchases, inviting you to go to their web-site and enter a "competition" for some free fuel.
Despite the fact that they have a pseudo-"opt-out" tick-box on the form, buried deep in the Contions of entry (Clause 24) is the following:
By entering the promotion, unless otherwise advised, each entrant consents to their personal information being entered into a database, and each entrant also agrees that the Promoter may use this information, in any media for future promotional, marketing and publicity purposes without any further reference, payment or other compensation to the entrant, including sending the entrant electronic messages.
So the reason for the "competition" ... harvest customer information and e-mail addresses.