Rewriting controller vs. adding a new controller to an existing module

Question

I'm looking at a community extension called Ambassador_Affiliate, and it's doing something that I have not seen before. I'm not sure what it's doing, so I'd like to check with the community.

In config.xml, there is...

<frontend>
    <routers>
        <customer>
            <args>
                <modules>
                    <Ambassador_Affiliate before="Mage_Customer">Ambassador_Affiliate</Ambassador_Affiliate>
                </modules>
            </args>
        </customer>
    </routers>
</frontend>

In Ambassador/Affiliate/controllers/AffiliateController.php, there is…

<?php

class Ambassador_Affiliate_AffiliateController extends Mage_Core_Controller_Front_Action
{
…


}

It looks to me the Ambassador extension is adding a new controller, instead of rewriting, its own controller to the Mage_Customer module.

If it is so, why would you do this as opposed to just defining a entirely new controller under Ambassador_Affiliate (beside wanting to use the customer frontname of Mage_Customer)?

If it's not, what is it doing?