The main reason to buy SP is that you expect it to go up in value (longer term). The rough analogy is SP being similar to buying a stock. When you buy stock you get to participate in shareholder votes, and you might also get other benefits (for example some companies offer discounts to shareholders). But the main reason you buy the stock is because you think it will go up in value.
RE: Steemit's Biggest Investment Problem - The Cost/Benefit Analysis Reward Curve