1. Not to mention that Portal enables you to implement security at various levels (pages/portlets/virtual portals/SSO) if properly configured against your user repository(ies).
Which beats having to build your own security framework to do the same thing with your pseudo portlets on a custom web app.
2. Plus various portals come with a content management components (ie Liferay and WebSphere, in my experience) so that you don't have to integrate any 3rd party tools and/or build your own.
3. Portals also have the capability to personalize.... but this feature I find is rarely utilized, but in certain environments, it may be useful.
There are many others... just can't think of any off the top of my head.