Pledging to contribute to a request’s bounty makes you an official backer of that request. Not only does your pledge increase the bounty and the likelihood the request will get fulfilled, you may be entitled to a fulfillment that only backers of the request receive.
For example, say a bunch of fans of a band want them to play in their town so they create a request directed at the band and crowdfund a bounty that ultimately entices the band to book the show. That band may choose to only give access to the tickets (or perhaps early access) to the backers of that bounty.
Ultimately, whether or not a backer receives something that a non-backer wouldn’t depends on the details of the request. However, all backers contribute to the ultimate success of a request.