Part of the human design is the pull towards the unknown. Working things out, discovering, learning. People are driven to explore and understand. And to master. People also tend to want new challenges after they have internalized previous challenges. Contests, freebies, and prizes are human ways of answering the need for the unknown and for the challenge.
Each person has a reason for entering a contest or vying for a prize or applying for a freebie. How does a freebie fit in? Because the wanting of a reachable-but-not-quite item or experience is a form of a challenge, albeit a minor one. The freebie-wanting is a way of working things out and mastering.
Also, people want entertainment. Another design feature! Contests, freebies, and prizes answer this need. Each in its own way entertains and interests.
The more one participates in contests and events that include prizes, the more one feels purpose, because the winning or placing or almost-winning or almost-placing satisfies a magnetic pull towards out-doing the unknown—changing the rules, creating a surprise ending.
Explore unknown (want ___ after ___ have )! Fill in the blanks and win a prize! Just kidding.