Ok, here's the truth. There's only one style that they are almost always necessary for: the infamous glam waves. Especially in the maui wind and humidity, this is a very difficult style to maintain throughout the day without extensions. Extension hair holds curl EXTREMELY well. The hair intertwines with your natural hair and the result is much more longevity.
With that being said, they can take any style to the next level. I'm talking about bigger buns, fuller braids, more volume and body! Even adding on a weft or two if you like a more natural look can help your style hold longer, and add just a little bit of fullness where it counts.
So, what extensions do you need? I love working with clip in extensions vs. Halo or semipermanent methods. Clip ins give me ultimate control in placement and volume, and are the most customizable. Luxy and Bellami are both wonderful luxury brands, and are a great investment if you are someone who wants to continue to wear them regularly after your wedding day. If you're on a budget, or just don't see yourself getting much use out of a new pair of extensions after your big day, I would recommend the set from Amazon that is linked at the top and bottom of this page. They're great quality for the price, and the one's I personally use for myself!