Well let's see... Shapeshift is completely anonymous. You don't need an account. You don't need to sit there and wait for small amounts of coins at a lower price (Shapeshift prices are usually lower than the average exchange price). And you can buy a larger block at the same price quickly. Customer service responds much faster than Poloniex, Bittrex, and other old school exchanges in my experience.
So to summarize, it is much faster, less hassle, anonymous, and you can save money.