How maven use It's plugins?

[秋秋]

HI,
    We use tag <plugin> int the pom.xml to add plugin in maven like this:
    <plugin>
  <artifactId>maven-site-plugin</artifactId>
  <configuration>
    <outputEncoding>GBK</outputEncoding>
  </configuration>
  </plugin>
   </plugins>

But ,if we don't configure this plugin in the pom.xml,maven can also excute
"mvn site:site",why?