waitForSelector("...", function success()/function fail() purpose?

Just starting out on casperJs and noticed the option of

waitForSelector("....", function success() {

},
function fail() {

});

My question is, what is the difference of this and using an if/else statement for successful code and errors? Are there benefits of one over the other? I've searched online, and on here, and haven't been able to find anything definitive.