Firstly, people write quality content that deserves upvotes in order to get money. you might get lucky and get a few pennies from a funny post, or ever a well thought out post. In the end, having more steem power allows you to make more as well. You can help boost your own post with your power which means other people will think its going to have even more votes (which ends up getting them curation rewards) so they will vote as well. Making quality comments on high dollar posts is also a way to make a little bit. Rewards are all based on content value added to a post, think of it that way