The key is that a portal solves many key problems that large, loosely integrated content providers face.
Portal builds upon J2EE, so theoretically, you could do everything a portal does in your own, custom built applications. But who's got the time? It's like
Struts of
JSF - you could do all that stuff on your own, but why bother when someone else, who is smarter and better looking than you, has done it for you?
Content management headaches, common look and feel, personalization, customization, ease of deployment...These are just some reasons to use a portal.
A portal solves some very big, very significant problems that clients face. But it's not for everyone. But when it fits, it's a good fit.
-Cameron McKenzie