sass extend example - JSFiddle

HTML

<div class="header">
  <h4 class="header-4">this is a header</h4>
  <span class="text">this is a span</span>
</div>

SCSS

// CSG
.header-4 {
  color: red;
  font-size: 1.2rem;
  font-weight: 100;
}

// if somebody decides to make the text same as header-4 by extending header-4
.text {
  @extend .header-4;
}

// And then somebody else decides to tweak the color just for the header-4 in this case by selecting and styling as below
// uncomment to see unexpected changes
 .header .header-4 {
  //color: blue;
}
 
 
 // So using mixin or classes directly is better to avoid this issue