Choosing your bridal hairstyle is something you would ideally want to do based on several factors: your dream wedding look, your face shape, your type of dress and neckline, environment conditions such as weather, amount of guests (or amount of hugs), time of the day, type of venue, among others.
So, why choose an updo? An updo, as well as any other hairstyle, can be personalized by adding some face framing pieces on the front, the textured desired (sleek, soft, messy, etc), playing with the direction of the pinning to add dimension and visual focus, and some more details.
So I would say the main reasons to choose an updo is to avoid getting hot and having the freedom to move, hug and dance without worrying of the hair getting messed up. Specially in hot weather, having your neck free of hair can be a game changer on your special day.