Well, I know you can sell USD and receive BTS to achieve buying BTS using USD.
I can do things like "to buy as many BTS using 60 USD" but what I can't do is "to buy exactly 10000 BTS for worst price of 0.006" if I "sell_asset xxx 60 USD 10000 BTS" when best price is 0.005.
For most (or almost all) other exchanges, you're specifying the exact quantity of BTS and the worst price you can accept.