Intranets connect computers within a set boundary, usually a company and its partners, over a LAN or WAN. Connections with partners takes place by connecting through a gateway. The main purpose of having an Intranet is to allow organizational data to be easily shared amongst its employees and other authorized users such as partners, in which it technically becomes an Extranet.
It is a given nowadays that most people who work with computers are familiar with the Internet and web-browsing, which makes use and navigation through an intranet easier and doesn't require much training.
As they use Internet protocols and technologies Intranets also allow easy communication and collaboration between employees that helps in increasing productivity as well as cutting down on expenses such as travelling.