why `this` is returning the object and why?

Question

i'm doing the freeCodeCamp challenge and i wanted to know the reason for the following:

var Window = function(tabs) {
  this.tabs = tabs; // We keep a record of the array inside the object
};

// When you join two windows into one window
Window.prototype.join = function (otherWindow) {
  this.tabs = this.tabs.concat(otherWindow.tabs);
  console.log(this.tabs)
  return this;
};

// When you open a new tab at the end
Window.prototype.tabOpen = function (tab) {
  this.tabs.push('new tab'); // Let's open a new tab for now
  console.log(this.tabs)
  return this;
};

// When you close a tab
Window.prototype.tabClose = function (index) {

  // Only change code below this line

  var tabsBeforeIndex = this.tabs.splice(0, index); // Get the tabs before the tab
  var tabsAfterIndex = this.tabs.splice(index + 1); // Get the tabs after the tab

  this.tabs = tabsBeforeIndex.concat(tabsAfterIndex); // Join them together

  // Only change code above this line

  return this;
 };

// Let's create three browser windows
var workWindow = new Window(['GMail', 'Inbox', 'Work mail', 'Docs', 'freeCodeCamp']); // Your mailbox, drive, and other work sites
var socialWindow = new Window(['FB', 'Gitter', 'Reddit', 'Twitter', 'Medium']); // Social sites
var videoWindow = new Window(['Netflix', 'YouTube', 'Vimeo', 'Vine']); // Entertainment sites

// Now perform the tab opening, closing, and other operations
var finalTabs = socialWindow
  .tabOpen() // Open a new tab for cat memes
//   .join(videoWindow.tabClose(2)) // Close third tab in video window, and join
//   .join(workWindow.tabClose(1).tabOpen());
// console.log(finalTabs.tabs);

why are we returning 'this'? is there any problem if we don't return the full tab but the array of tabs itself?

Also, do we HAVE to replaced this.tabs? what type of role does it have by replacing it?