I often get asked why people need a wedding planner. They say, "I've already done the planning, why do I need you there on the day-of? My site manager takes care of everything." A well-orchestrated event runs a lot like a tightly written Broadway production. The best way to explain how this happens is to give a specific example.
Obviously the wedding ceremony requires a lot of orchestration but let's take something simple like the cake cutting. Here is how the cake cutting went at a recent wedding I produced:

15 minutes to cake cutting
Planner finds caterer manager
PLANNER: "We're going to do the cake cutting in about 15 minutes. Please get the toasting flutes the bride and groom brought in specially for the toast and fill them with the special bottle of champagne we stashed in the back. Can you place these on the cake table with the cake knife and server the couple bought, two plates, two forks and a napkin. Please make sure someone is standing by to remove the cake to take it in the back to cut"
CATERER: "Got it, I'll put my people on task"
10 minutes to cake cutting
Planner finds photographer
PLANNER: "We're going to do the cake cutting in about ten minutes, do you need anything?"
PHOTOGRAPHER: "Actually yes, there is that spotlight shining on the cake, it's going to wash out my pictures. Is there any way we can turn it off so we just have soft candlelight for the cake cutting?"
PLANNER: "Yes, I had the lighting company place that light on a separate switch so that when the cake was removed I could turn it off. Let me get the remote and have it ready for the cutting."
7 minutes to cake cutting
Planner checks in with the Bride and Groom
PLANNER: "We're going to do the cake cutting in about five minutes, are you ready?"
GROOM: "Actually, I have to use the bathroom first"
PLANNER: "Sounds good, when you get back we will do it then"
2 minutes to cake cutting
Planner waits for the Groom to get back... go check-in
PLANNER: "Are you ready? We're going to do the cake cutting after this song"
BRIDE: "All set"
PLANNER: "Great, don't stray too far from the cake table."
1 minute to cake cutting
Planner checks-in with the band.
PLANNER: "When this song is over, we're going to do the cake cutting, can you announce to the guests that we're going to cut the cake and play a fun, upbeat song in the background for the ceremony? I know it's time for you to take a break, so after the cake cutting you can go in the back for a break during dessert"
BAND: "Thanks, that's great. We'll play something fun"

Cake Cutting
Band announces cake cutting. Planner cuts lights over cake. Caterer in place to remove cake. Photographer in place to shoot action. Planner brings Bride and Groom over to cake
PLANNER: "Just make two even diagonal slices in a pie shape in the bottom layer and remove the piece with the server".
Cake is cut. Band plays "Pour Some Sugar On Me". Photographer captures the couple laughing and singing along. Caterer clears cake to back to cut into slices. Servers pass slices. Planner goes to the back to get the Bride and Groom's cake server and toasting flutes to wash them and pack them up in their original boxes for the end of the night.

Here is what I've seen happen at weddings without a planner:
Band announces cake cutting and takes a break from playing. Groom is in the bathroom, awkward silence while guests look for the Groom. Bride and Groom ready to cut the cake but the server and knife are missing and there are no plates. Caterer rushes around to get a knife for them to use, doesn't realize there was a special serving utensil and champagne flutes. Awkward silence while guests hungrily stare at bride and groom cut the cake without music. Bride and groom not quite sure where or how to cut. Awkward laugh. They attempt a messy cut. Bride and groom make "mmm" noises to fill the silence. Photographer does his best to take photos but they don't come great because there is a glare and the bride and groom are flushed with slight embarrassment. Toasting flutes go missing at the end of the night.
When a wedding goes smoothly no one notices anything happening behind the scenes but without a planner the simplest task can turn into an awkward moment and interrupt the flow of the event.
All cakes from Martha Stewart WeddingsLabels: advice