One use of “CSS class” for menu item is when you wish to style it differently from the others.
For example :
Here, the “Contact Us” menu item has class of “CTA-menu”.
And in the custom css, the styling is defined as:
.CTA-menu a {
background-color: #45b6f5;
color: #fff!important;
border-radius: 3px;
margin-left: 1em;
font-weight: 600;
transition: .24s ease-out
}
.CTA-menu a:hover {
text-decoration: none!important;
background-color: #3594c9
}
BTW, I noticed this from one of the showcased site