If you're an investor, you want to see a return on your investment. Your most efficient way to do that is via self-voting.
Also, votes get you attention on Steemit. Some people will not even view your post unless it has at least 1 vote to begin with. Therefore, it's advantageous to self-vote for more views.
It's all just a part of the way the platform was designed.. with altruism at heart.