People will go out of their way if motivated to do something. Their intention drives their actions and keeps them focused on completing the task (Ajzen, 1991). A person intent on earning free swag will take time away from other activities and input My Coke Rewards codes into the Coke website every time they drink Coke products. When someone is intent on completing an activity, they are motivated to complete it and focus their attention on it.
Someone determined to complete an activity will also not be as attentive to other stimuli. For example, if people intend to get to Space Mountain at the back of Disneyland Theme Park right when the park opens (motivated to avoid long queue lines), anything placed in their way, such as promotional advertising, will likely be ignored. They could also neglect to notice they forgot to put their 3-year-old daughter back in the stroller on Main Street USA, and now they have to go back to find their child.
Intention is more complicated than rewards programs and helpful behavior. Dan Pink discusses this in his TED talk, The Puzzle of Motivation.