Why was the us capitol building built?

i am doing a report on the us capitol building and i can;t seem to find this answer but maybe you people out there can help me. Why was the U.S. Capitol building built? Also i am going to add 2 more questions you might also be able to help me with. Someone told me that there is a library inside the Capitol building. Is that true? and last question... Can someone give me some interesting facts about the U.S. Capitol Building?