>I've not figured out this cross-posting feature yet, but I see you can re-Steem your community posts to your feed. I assume in that case they only count as one post.

Yes, so this add a community post to your own blogroll.

Cross-posting creates a new post and therefore, a new open reward opportunity. The benefit is, you can post someone else's content and take a small slice of the author reward, the community gets a small slice, and the original author takes 90%. It is a powerful opportunity to incentivize sharing and give a lot of exposure for authors, plus additional rewards.