On the actual Wedding Day the couples are rushed from place to place barely taking a breathe. You're enjoying every minute but you can hardly take it all in. Many times the wedding pictures are the first time the couple even no what really happened at their own wedding. It's natural that everyone would want some of your time on that most special of days.
For the engagement photos, the time stands still. It's just the couple and the photographer. The couple can really relax, not worried about their next hug or handshake or talking to someone that they haven't seen in years.
For the engagement photos the couples can just relax and have fun. No one worries about spilling anything on a ten thousand dollar dress or ruining the rented tux. Instead, you can run on the beach and get in with the waves. You can roll on the grass or feed each other from the other's ice cream cone. You can wear what you want from casual to formal to any combination of the two. You name it- it's your special, almost intimate, sharing of the two of you when no one else is around.