Does initializing a variable with true or false establish the direction of the content in Javascript?

enter code here
   <script>
     var switchDirection = false;
      function doAnimation() {
        var divAdvert = document.getElementById("divAdvert");
           var currentLeft = divAdvert.offsetLeft;
            var newLocation;
           if (!switchDirection) {
               newLocation = currentLeft + 2;
               if (currentLeft >= 400) {
              switchDirection = true;
              }
          } else {
                  newLocation = currentLeft - 2;
                  if (currentLeft <= 0) {
                  switchDirection = false;
                  }
          }
             divAdvert.style.left = newLocation + "px";
       }
     setInterval(doAnimation, 10);
   </script>

I want to know why a new variable with the value of false has been declared in the beginning of the function.