Question on Module.xml

HI All,

  I somehow got confused along the way. Please clarify my doubt. 
  If I am importing the build.xml in my centipede file, and have specified in the ant tasks all the classpaths to the different third party jars etc needed by the project, then why do I still need to specify the same jar files in the module.xml. What I am asking is that what purpose is served by specifying the depend project in the module.xml. I can run the junit test through the ant build file itself, so the classpath passing is adequate in itself.
  Is it that the cents such as javasrc, junit etc also need to know about the 3rd party jars and hence we absolutely must specify the depend project attributes in the module.xml?
  Please clarify. Thanks,

Regards,
Nikunj