Light. You can't take a photo without it. Not all weddings are within the hours of daylight and not many venues provide a great amount of light indoors. Take your living room for example. At night you probably have at least one light on, and for you that is enough to read your book or watch TV. For cameras this isn't enough light to take a quality image.
By quality I mean having enough details in the highlights and shadows and not introducing grain into the image. For most images I would like as little grain as possible, not because it looks 'grainy' but it reduces the quality of the colours of the scene.
At a recent wedding I encountered the darkest venue I have shot in. A winter's day with little light within big rooms. To compensate for this I used flash to add my own light. The below video will show how I took the shot and how it affects editing.
Different from the pop up flashes on compact cameras and the bright bulbs of iPhones, the additional flash used here is more powerful and directional, meaning I bounce the light to illuminate more of the space and provide more flattering light on the people involved.
I would say the majority of photographers prefer natural light. But, it's not always available and flash allows you to get images you would otherwise miss out on. Without it photos become blurry as your camera takes longer to capture the shot, more time = more shake.
Something to discuss with your photographer beforehand is flash during the critical moments of the day, i.e. ceremony and first dance. If not used, most images will probably end up black and white, to remove the washed out colours, but big flashes aren't subtle either. Each to their own.
This wedding was a busy but fun experience. Having the equipment to help tackle challenging environments means you approach the job with confidence. Without a doubt, this image would not have been possible without flash. It's not a studio, I don't have 3 lights set in exact positions with varying degrees of power. I had one light that helped my capture a moment, people enjoying themselves.
If you have any questions about this image or the editing involved, then please get in contact.