When considering a Corporate Portal, the main objectives can be more-fold.

The basic oppertunities however are clear:

• Easier collaboration between employees, business units, supliers and even customers.
• Huge time-saving oppertunities = money saving.
• Faster and cheaper deployment of new and existing applications.
• Realtime dashboard-like management information

Intranet ROI: Executive Summary

As with any critical business system, an intranet or portal must be delivering measurable performance and remain accountable to the investment. If the site's value is not being measured, then it risks failing the needs and demands of employees and management.
When asked how executives determine whether an intranet or portal is delivering value, they typically point to reducing costs and improving productivity.
Of course, IT budgets are increasingly tied to company-wide business and strategic initiatives. The cost justification for any expenditure, including IT must have a clear bottom line that answers the question, What’s the payback?
In recent years, business challenges and subsequent solutions/applications have been the driving force behind intranet investments. They will continue to play this role in the future. Executives will make investments in intranets and portals if such investments spur growth, cut the cost of operations, and/or help enhance the customer experience (retaining and building the customer base). The challenging economic climate of the past few years means that more and more organizations will allocate dollars to IT systems and applications that can demonstrate a measured solution and ROI to a business problem or provide enhanced service and growth.
Read the whole article about ROI of an intranet here.