You need a photographer if you want to have professional-quality photographs taken of you before, during, and after the ceremony to remember your day for the rest of your life. Of course, like pretty much everything else about a wedding, you don't actually "need" a photographer. You could get married at city hall with no ceremony at all.
It is important to have someone taking pictures who is not involved in enjoying the wedding and someone who will take neutral pictures. Someone who has ties to people in the wedding will take more pictures of those people and not of others. A professional can compensate for odd colors and lighting in an uncontrolled location.
It's not required to hire a photographer for a wedding since guests will take pictures, but if you want someone to focus on you and your spouse and get professional, candid photos to remember the day by, then a photographer is the way to go.
In short if you need high quality, formal portraiture at your event you need a pro! You wouldn't ask your neighbor to fix your shinny new laptop! Would you?