Unlike Twitter, which is for the general public, Yammer is focused on companies, and only users with the same email domain can be part of a given network. It also takes a slightly different approach with updates. Twitter asks “What are you doing”? and Yammer asks “What are you working on?”
Yammer also seems to be a “mini-facebook” since every employee has a profile, which could be useful for large companies. Organizations with remote users may also find this feature helps remote users feel more connected to the organization.
Having a knowledgebase with past conversations has the potential to reduce email conversations. With conversations in one place, it could serve as a public email system for users and possibly reduce email overload.
Since it is web based, there’s no need to host or maintain the service on a company server. The basic version is free for those who simply want to try it out. If it turns out to be something your company wants to implement, there are several different options for upgraded service.