A resteem is like retweeting in twitter. It is a form of sharing content.
I shared the content of this author to all of my followers (who are almost 3500 at the moment), so that the post can be noticed by more people.
The benefits are not direct. In fact, if a post is crappy, there will be no benefits, and maybe even some negative effects. All it does is expose the content to more people. If the people like it, the author benefits. If they don't - he might even get flagged.