Casting To Boolean in If statement

I have recently read some code that uses !! to convert a variable to boolean for evaluation in an if statement. This seems somewhat redundant to me as the variable would be evaluated for its boolean value anyway. Are there any performance benefits to doing this or is it for better browser support?

Sample Code:

var x = 0;
var atTop = x===window.scrollY;
if(!!atTop){
   alert("At the top of the page.");
}