One approach that works for internal communities is when the organization needs to know or needs other people to know. It is difficult to find out which sales teams working on a contract since the contract appears in the system after it is signed. Having each team member add that information in the expertise section means that a search will find them. Also if an organization wants to make sure people can find the expert, they will make sure the information is in the profile.
Find the compelling business need and the people that need to know or to make sure others know.