Investors due to the fact they have the highest stake in the system, developers in other to make their project come true they need the help of an investor who believes in what they are doing, in otherwords they need finance.
Along the line, certain bargain or due to help rendered by the investor, the developer might find it fit to reward him/her upon completing the project, which acts as more reward to the investor.
Also your steempower determines the amount you get from curation and as an investor you stand to benefit more when ever you curate a post be it made by a developer or not.
The aspect of contents creator, we are all actually content curator.
Even looking at investors, you can classify anyone that has steem or sbd an investor and it all comes down to how much they invested to get a good return be it from curation reward or other means