The points of user portal are (1) single point of access and (2) personalized user experience.
Portal is 'convenient' for users, and 'requisite' for the site administrator/manager.
For a user, the portal is the starting point (gateway, or root menu) of information and services adapted to her/him, and the point where s/he can overview the personal site map. It brings her/him in efficient environment for job performance. For example, reducing the need to move around is an effect of this architecture.
For the site administrator/manager, the portal is an architecture of personalization equiped with user authentication (control of access permission).
Common user interface may provide consistency and unified user experience. On the other hand, it is common that at personalized web site users are allowed to customize. Thus the design of user portal stands on a balance between unified environment and diversity of individuals.