You try to unfairly increase the upvotes on your submission, by getting people who don't read HN to register and upvote your submission. Or, you create a bunch of HN accounts, and log into each one and upvote your submission.
Getting on the front page of HN can drive a lot of traffic to a site, which can turn into money, so there's a financial incentive to game the system.
Getting on the front page of HN can drive a lot of traffic to a site, which can turn into money, so there's a financial incentive to game the system.