Advantages

Exchange large volumes of data using Electronic Data Interchange (EDI)
Share product catalogs exclusively with wholesalers or those "in the trade"
Collaborate with other companies on joint development efforts
Jointly develop and use training programs with other companies
Provide or access services provided by one company to a group of other companies, such as an online banking application managed by one company on behalf of affiliated banks
Share news of common interest exclusively
http://en.wikipedia.org/wiki/Extranet