First, what's the obvious reason you need an engagement session? To document the newly-engagement excitement, of course! But you can do that with anyone...or with a cell phone for that matter. I want to share why you need an engagement session with the person who will actually be capturing your wedding.
To increase your confidence in front of the camera.
Getting in front of the camera for the first time is nerve wracking. Once you do it though, you'll learn what feels comfortable, and how that photographer will guide you into poses that are flattering. And once you see those photos, and realize how they were able to capture you, you won't feel so nervous the next time. It's such a good idea to get this out of the way before the wedding.
To connect with your photographer.
As if your wedding day doesn't have enough going on, throw a stranger into the mix, being in your space the entire day. They're there for all the moments, telling everyone what to do. Awkward. It's well worth taking some time to hang out with your photographer before the day-of since that is the vendor you will be spending more time with on your day than any of the others. This is also why it's so important to choose a photographer based not only on their style of photography, but also on whether or not you clique with them and actually like them.
To teach your photographer how to capture you.
Every relationship is different, and every couple interacts with each other a bit differently. Your photographer needs to get to know you a bit to see what kinds of interactions are going to look natural for you. They also need to be able to read your smiles and differentiate the fake ones from the genuine ones. An engagement session is the perfect time for your photographer to try out different things with you and find the things that work the best for you as a couple.
Removes any uncertainty in what you'll be getting.
Now, before I say this, I'm very confident in my ability to turn around a gorgeous wedding gallery after all the years I've been doing this. But if your photographer is one you've never worked with before, there might be the lingering question in your mind of how they'll make you look in photos. Do you want to find out on photos that can't be redone? An engagement session is kind of like a trial run. You'll be able to see how the photographer directs you, and what kind of final product they'll deliver. That way, you won't have one bit of doubt on the day of the wedding, which leaves you with one less (major) thing to be stressed about.