A normal swap is an order to swap tokens at market rate. The rate is not specified at the time of placing the order.
A limit order is an order to swap tokens at a rate that is guaranteed to be higher or equal to a specified rate.
The advantage of the normal swap is that the order will definitely be executed at the prevailing market rate, however, the trader might end up paying slightly more or selling at a slightly lower price. (i.e slippage)