Yes, I won a digital camera last year in a photographic competition. The reason sites run competitions is usually to get more contact email addresses and you can expect to get further contacts from the website. The competitions on some websites are sponsored by the manufacturer of the prize and just gain extra publicity for their product. Since a minority of sites on the www are not 100% trustworthy, there are almost certainly some competitions that are just scams.
You have to use your own judgement as to whether to enter or not.