Why are some authors listed more than once, and under different agents?
This is because the listing shows current and past agents. Knowing that an author was once a client is just as valuable as knowing who that author's agent is now. The agent's tastes probably didn't change, the author just moved on.
Where does this information come from?
Most of it is gathered from the authors and agents websites, and a lot is provided by QueryTracker users.
I know of an author/agent relationship that is not listed here. How can I add it?
On the "Clients" tab of each agent's profile page there is a form for adding known clients.
Why does it help to know who reps whom?
When seeking an agent, it is advisable to target agents who represent books similar to yours. For this it helps to learn the authors and books which the agent represents. It is also useful to state in your query letter why you are approaching a certain agent. Being able to state a similarity to past clients helps with this.