So whats this first look thing everyone is taking about? Well, its exactly what it sounds like! Its setting up with your photographer to make sure that they capture the expression and admiration of a groom seeing his bride in her wedding attire for the first time!
Typically for civil ceremonies, this moment will happen during the ceremony as the bride walks down the aisle, but first look shoots are still often a good idea because doing formals on a separate day is becoming more popular, so seeing the bride in her wedding attire is a lot less ceremonious when you just show up to your photo shoot and see each other. If you will set it up with your photog before hand, they can position the couple so that they are able to get a lot of fun, beautiful shots leading up to that exciting moment, and most importantly, capture the grooms look of adoration at seeing his sweetheart all dolled up and looking like a bride!
For LDS weddings, the first time a groom sees his bride on the wedding day, she isn't typically in her wedding gown, as temple weddings require certain articles of clothing be worn for the sealing. They first time he sees her in the complete get up, gown and all, they are usually in the hall way being ushered by temple workers to the doors where they will then exit the temple and then be greeted by family and friends! It doesn't really give that moment any time to really 'land' because they are then rushed out to door to be greeted by family and friends.
If those aren't reason enough, just take a look at the expression on these grooms face when they saw their brides for the first time! Don't you want pictures like this?!