The user needs to know whom the service (uPortal/Sakai/Kuali Student) thinks it is talking to. Specifically they would like to be able to ask the service:
The user has logged in to the service, but may be unsure of their status. Querying the "Who am I" service will assure them that the service knows who they are. This is especially important to someone who has different credential sets for engaging the service in different ways.
Services commonly attempt to fulfill the requirement by displaying a welcome message at the top of the screen: E.g. "Welcome Fred Flintstone" or "Hello mburns" or "Greetings 31415927". Some users do not want their personal identifiers displayed in this way.
Users frequently engage services through personas. (We're using the word persona with its identity management meaning - briefly, it means the role, or set of roles the user adopts for the session - e.g. I'm logging in as a student, I'm logging in as a parent, I'm logging in as an instructor.) This means that the answer to th question "Who am I" does not always have the same answer for a particular user. If the user has authenticated through a federated identity provider, the answer may be something like "You're a librarian at Enormous State University". Bur even if we don't consider such extended cases, it's very useful for the user to be able to confirm their identity.