To try and look like real users. Then they sell out upvotes to companies wanting to promote their own posts.

In order to interact in many reddit communities, a certain amount of karma is required. The idea behind this was that it would be harder to just make a million new accounts and start spamming posts. The reality is that it just means that anyone who wants to spam or scam on reddit now has to build a bot infrastructure that can make itself seem legitimate by artificially interacting with the site to gain karma. The bots do this by interacting with the site, making posts to gain karma, etc.