Why sould we specify dependencies as we define a driver module in JBoss AS 7?

That's how is driver defined in the official JBoss documentation

<module xmlns="urn:jboss:module:1.0" name="com.mysql">  
  <resources>  
    <resource-root path="mysql-connector-java-5.1.21.jar"/>  
  </resources>  
  <dependencies>  
    <module name="javax.api"/>  
  </dependencies>  
</module

I didn't get why should we define the dependencies tag? What is it responsible for? Of course, We should define a path to a driver, it's clear. But the <dependencies> confused me. Could anyone explain it to me?