In addition to what always turn out to be more intimate moments, there are practical elements to doing a first look. Just listing off a few things -
-You see each other when hair and make up are freshly done.
-It's unscripted, so you get to react and do whatever you want. (Not so much an option in church ceremonies) Hug each other. Do a little "we're getting married dance." Whatever, this is the time for it.
-More time for pictures! Seeing each other before the ceremony allows you to build in more time for some of the great creative portrait shots you're probably hiring me for. We'll also get more time to do the fun and creative pictures with your bridal party.
-You get more time with your friends and family. You've probably got lots of people that have come together for this party that you've spent a ton of time planning. I want you to spend time with them! Besides, if you know that there are a bunch of people enjoying your party without you then you'll probably be distracted and won't take as good of pictures anyway. If we've done pictures beforehand, then you get to spend more time with them and just having fun.
-Did I mention that it's more private? I can't stress enough that between first looks and the more traditional "first look at the ceremony," the private, pre-ceremony interactions tend to be incredibly more genuine and touching. People are simply more "real" when there's nobody watching.