Such sites are like practice exercises in Accounting 101. Building them makes an excellent, low-risk training ground for webmasters. But I don't know of any federal jobs that require skills gained by playing Solitaire or Tetris. Intranets can also be good for promoting intra-organizational understanding. Many federal employees are only dimly aware of the scope of their own agencies.
Intranets can make the entire agency accessible to every employee.
Intranets are also nice for organizations interested in Web technology but have too little information of interest to the public. Internet sites are onstage 24 hours a day, seven days a week before the entire world. Some visitors, aware that their tax dollars paid for the sites, are eager to express their opinions on the utility and usability of the pages they see. By contrast, intranet sites have a more limited audience. In the worst case, it may be only the managers who commissioned them.