C.C.
The agent wants to see these documents--which you would write once she became your agent--BEFORE she takes you on. If your manuscript is great, but your pitch and synopsis aren't anywhere near good, that will affect her ability to get your manuscript accepted by a publisher.
Consider it a form of your own control over your work. You would want to write these documents to make sure they fairly represented your work. And many publishers will request these documents either with a full manuscript or BEFORE they are even willing to read the manuscript itself.
If you doubt the agent, look up the agency on Preditors and Editors online. They are the most up-to-date source for which agents and editors are shady. And drop any prospective agent who asks for money up front (for copies, etc.) before a book deal is made. Agents for books are just like Realtors... they get money when you do, and not before.