Honestly, most of the time it is not about either. It is about boosting site hits, which thus increases the marketability for advertisers, or at the very least increases the chances that some of those people are going to look around and order from a retail site that offers the contests. It's also about page ranking within the search engines. People post the links on forums or on their own personal websites, which are relevant links to the search engine bots, and thus the website looks more valid for the relevant keywords and is placed higher on the list.
The legitimate contests, IMO, either have the results hidden, or only allow one vote per month per IP address or email address.