Intimacy - When a first look is done before the ceremony, very frequently it's only you, your partner, and the photographer. This makes for a much more personal moment than it perhaps would be if you're seeing each other for the first time at the ceremony.
Practicality - Wedding days are stressful and full of potential for things to go wrong or run late. Blocking out time before the ceremony for a first look and other portraits will make sure you have time to create the beautiful images you want from your day. Your photos are something you have forever, and this way they aren't hurried or cut short because of time constraints. Not to mention, hair and make-up is perfect and fresh.
Perfection - Speaking of things being perfect, doing a first look before your ceremony will allow your photographer to create the most beautiful image possible. During wedding ceremonies photographers are often hindered by factors such as lighting, seating, and general layout. Any photographer worth their salt is able to work around these obstacles, but if given the chance to stage a first look, your photographer will be able to give you something truly special.
Hospitality - Doing your first look and couples photos (which are arguably the most important) before the ceremony cuts down the gap between the ceremony and dinner service. This way your guests aren't left hungry and wondering while all your photos are completed.
Nerves - Seeing each other before the ceremony will greatly calm any wedding day jitters you may have. It's your wedding day, you should be spending it together! Doing your personal photos before your ceremony gives you extra hours to enjoy one another on your special day, hours that would have otherwise been spent apart.