What's the purpose of dropdown-toggle class in bootstrap dropdowns?

Question

Dropdown menus work perfectly with or without the dropdown-toggle bootstrap class being applied to <button> element,so why use it in the first place?

Answer 1

The dropdown-toggle class adds the outline: 0; on :focus to the button, so when you click on the button it will not have the surrounding blue border of the "active" element.

Check the next two bottons:

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>

<div class="container">
  <h2>Dropdowns</h2>
  <p>The .divider class is used to separate links inside the dropdown menu with a thin horizontal line:</p>
  <div class="dropdown">
    <button class="btn btn-default dropdown-toggle" type="button" data-toggle="dropdown">Tutorials - no border
      <span class="caret"></span></button>
    <ul class="dropdown-menu">
      <li><a href="#">HTML</a></li>
      <li><a href="#">CSS</a></li>
      <li><a href="#">JavaScript</a></li>
      <li class="divider"></li>
      <li><a href="#">About Us</a></li>
    </ul>
  </div>
</div>

<div class="container">
  <h2>Dropdowns</h2>
  <p>The .divider class is used to separate links inside the dropdown menu with a thin horizontal line:</p>
  <div class="dropdown">
    <button class="btn btn-default" type="button" data-toggle="dropdown">Tutorials - with border
      <span class="caret"></span></button>
    <ul class="dropdown-menu">
      <li><a href="#">HTML</a></li>
      <li><a href="#">CSS</a></li>
      <li><a href="#">JavaScript</a></li>
      <li class="divider"></li>
      <li><a href="#">About Us</a></li>
    </ul>
  </div>
</div>

Answer 2

It adds the following CSS properties, but they impact when dropdown button's content is displayed:

It's basically some button inner box-shadow when .open, as well as color, background-color, border-color and outline (on :focus) removal. Here a comparison between the two:

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet" />
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<!-- Single button -->
<div class="btn-group">
  <button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
    With .dropdown-toggle <span class="caret"></span>
  </button>
  <ul class="dropdown-menu">
    <li><a href="#">Action</a>
    </li>
    <li><a href="#">Another action</a>
    </li>
    <li><a href="#">Something else here</a>
    </li>
    <li role="separator" class="divider"></li>
    <li><a href="#">Separated link</a>
    </li>
  </ul>
</div>

<!-- Single button -->
<div class="btn-group">
  <button type="button" class="btn btn-default" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
    Without .dropdown-toggle <span class="caret"></span>
  </button>
  <ul class="dropdown-menu">
    <li><a href="#">Action</a>
    </li>
    <li><a href="#">Another action</a>
    </li>
    <li><a href="#">Something else here</a>
    </li>
    <li role="separator" class="divider"></li>
    <li><a href="#">Separated link</a>
    </li>
  </ul>
</div>

Difference tested in Chrome, Opera & Safari: