title: Why modelVersion of pom.xml is necessary and always set to 4.0.0?
body: <p>I have noticed that Maven's <code>&lt;modelVersion&gt;&lt;/modelVersion&gt;</code> of <code>pom.xml</code> is always set to <code>4.0.0</code>.</p>
  <p>Can you please help me understand what is the importance of this tag and why it should be set to <code>4.0.0</code>?</p>