This is an open question about the domains for SharePoint apps. On Technet: Configure an environment for apps for SharePoint (SharePoint 2013) we can read the following:
You must configure a new name in Domain Name Services (DNS) to host the apps. To help improve security, the domain name should not be a subdomain of the domain that hosts the SharePoint sites. For example, if the SharePoint sites are at Contoso.com, consider ContosoApps.com instead of App.Contoso.com as the domain name.
Does it apply to SharePoint Online? Well, apparently not :) So why should we do it on premises?
As we all know, sharepoint.com is used for our Office 365 tenancies and for apps.