JavaScript 这个代码,返回值 return false;是为了什么?

别的理解的差不多了,就是不懂为什么要有一个 返回值 false
他是干什么的?

if (window.Event) 
  document.captureEvents(Event.MOUSEUP); 

function nocontextmenu() 
{
    event.cancelBubble = true
    event.returnValue = false;

    return false;//★★★★★★★★★★★★★★
}

function norightclick(e) 
{
    if (window.Event) {
      if (e.which == 2 || e.which == 3)
       return false;
    } else if (event.button == 2 || event.button == 3){
           event.cancelBubble = true
           event.returnValue = false;
           return false;  //★★★★★★★★★★★★★★
      }

}

document.oncontextmenu = nocontextmenu;  // for IE5+
document.onmousedown = norightclick;  //for all others