A lot of my friends ask me the difference between portals and web-sites. Typically an organization would have both - a company website and a portal. I found a good explanation here.
Portals typically have authentication and provides us information based on who we are. Personalisation and customization are the hallmarks of portals. So we require portals because:
Different roles require different information. Someone from grounds and buildings needs different info than the chair of computer science. (Customization) Different people with the same role work differently. (Personalization) Efficiency - people get directly to the info they need. (Work Flow) Customization insures they don't miss anything.