For example: 
You have a replicated landing page you share with someone and your landing page URL is username.myexpresssite.com/pages/landing_page/. That long URL does not look too pleasant for marketing purposes.  So, we give you the ability to take any page you create and assign it to any domain you have registered in you account.

When you register domain.com and assign username.myexpresssite.com/pages/landing_page/ to that particular domain, you're done.  You can now send advertising to your new domain and it will pull in the content from username.myexpresssite.com/pages/landing_page/, therefore...no forwarding or "ugly URL" needed.

You can also assign any existing pages in your account  to a particular "folder" on your domain.  As an example...let's say you have a page called "username.myexpresssite.com/pages/landing_page/tutorials" in which you want to send your visitors to in order to provide them training videos, software, etc..  You could use a separate url if you wish and register MyTraining.com.  Have your "tutorials" page assigned to MyTraining.com.  Your visitors could then be sent to MyTraining.com and would automatically include "username.myexpresssite.com/pages/landing_page/tutorials" template and content.