When you hire a professional in any field you're getting someone who's "been there and done that" many times before. Take, for example, a wedding. A lot is happening from the time the bride and groom are getting ready, to the arrival of the guests, the walk down the aisle, the ceremony, the presentation of the bride and groom, the celebration afterwards, and the reception. Now think about the hundreds of small but magical moments along the way - a tearful mother helping her daughter into her wedding gown, the bride and groom looking into each other's eyes while they exchange vows, and their first dance together at the reception. Also remember many of these moments happen in difficult conditions where it can be hard to get a clear, well exposed photo.
An experienced photographer knows where to be, when to be there, and how to use his equipment to capture the moments and emotions that really tell the story. When you're making the decision to hire (or not) a professional, ask yourself if you will be happy if some important moments are missed or not captured as you would like to remember them.