It is actually not about serving content. Custom domain makes the fail over much easier. For example, someone uses webscript.io as his app backend. If webscript

It is actually not about serving content. Custom domain makes the fail over much easier. For example, someone uses webscript.io as his app backend. If webscript.io goes down for whatever reason, he can simply point the URI to a backup server(provide he has one) if he uses a custom domain. Without custom domain all he can do is wait.