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if you're going to buy 10 dollars worth of steem, it doesn't matter if the price is high or low, you always have 10 dollars to invest

Right. But we aren't talking about small amounts. If you believe in the project. And you want to power up your steempower to a high level. So when you vote you can upvote others and make them help money. Then you'll want the price to be low so you can buy in bulk.