We don't just take photographs of you and your partner, we capture the mood, the people who are celebrating the day with you, and the little details that you worked so hard to get just right. When a photographer arrives earlier than when you planned to get ready, they are most likely either capturing some of the details of your shoes, dress, or the rings, or they are getting the room ready (lighting wise) making every photograph look just right.
Make sure your photographer discussed the timing of the day with you well before hand, and sends a detailed time schedule as to where he / she will be when. This way you know what to expect from your photographer on the day. The time schedule should include where his / her assistant will be as well so that you and your partner will know when to be ready for them.
The most important thing to remember through out your wedding, is relax, enjoy every minute and don't' worry about the cameras, the photographer is there to capture the moment, not always a posed composition.