I feed frozen/thawed. Is that what you are referring to? Pre-killed is when you take a live prey and dispatch it before feeding it to the snake...
The biggest advantage to feeding Frozen/Thawed is that you can buy them in bulk and save money. Another advantage to feeding dead prey over live is that you don't have to worry about your snake getting bitten. Not to mention if the snake doesn't eat, now you have a live rat to take care of....