For what? To engage in the community as a whole and to encourage others to as well, or to get your community noticed by people browsing the different filters (new, hot, active, top...)
For why I'd want to post? It would depend on what I'm posting. For example, if it was a meme I thought was funny and it got a lot of upvotes, I'd feel good knowing that other people found it funny, too
If I post looking for a conversation about a topic or at an answer to a question, I'd be excited for some upvotes but a little let down if no one actually responded
Also there's no incentive to be a karma whore on reddit either.. well except their harebrained idea to monetize whatever new award system is coming
There's lemmynsfw! And also the same answer for any community growth: post content ๐