Question

I see no situation where I need this

(function(param){
alert(param);
//do something
})("derp");

istead of this

alert("derp");
//do something

EDIT: ok, thanks everybody, i think i got it.
so if you have this:

var param = "x";
var heya = "y";

(function(param){
    alert(param);
    //do something
    })(heya);

the global variable “param” will be ignored in the scope of the anonymous function?

Answer 1

How about this scenario?

Example #1 – Uses an Immediately Invoked Function Expression that closes over a single variable and returns another function. Resulting in a beautiful, encapulated function.

var tick = (function () {
   //Example of function expression + closure
   var tock = 0;

   return function() {
        return ++tock;
   }
}());

//It is impossible to alter `tock` other than using tick()
tick(); //1
tick(); //2
tick(); //3
tick(); //4

VERSUS

Example #2 – Uses a Function Declaration w/ a global variable

//Unnecssary global (unless wrapped in another function, such as jQuery's ready function)
var tock = 0;

function tick() {
    return ++tock;
}

tick(); //1
tock = 4; //tock is exposed... and can be manipulated
tick(); //5
tock = 6;
tick(); //7

It is a contrived example but still a real case scenario in situations where people may want to generate consecutive UNIQUE ID’s with no possibility of collision.