Answers for "why we use first child"

why we use first child

<style> 
    .container:first-child{
     background-color: blueviolet;
    }
</style>
<div  class="container">
        <ul id="nav">
            <li>Home </li>
            <li>About</li>
            <li>Servicess</li>
            <li>More About Us</li>
            <li>Contact Us</li>
        </ul>
    </div>