People have different forms of trading. If you look at Chinese exchanges with zero fees, then it is perfectly suitable for ultra fast traders benefiting from even the slightest price change. If you trade with 3BTC and you sell these coins at $3 profit each, then it will be $9 which is peanuts. But if you do that with at least 500BTC, and then several times a day, then do the math. It will make you thousands of dollars every day. Doesn't sound all that bad, right? If you do that 100-150 days per year, or even a full year, then you'll be a very happy person where you can quit your day job as this will be your main source of income.