Why not just a fast shutter speed to freeze motion? That's what I always think, but there are good reasons to use a flash or speedlight to do it instead. Take a look to see what they are.
I've always lived next to the coast, growing up near the southern beaches of Sydney and more recently in the far south-west of Japan. That's meant that I have always loved the outdoors and the ocean, particularly surfing. So, when I got more serious about photography during my university years, it was only natural that I would gravitate towards landscape photography and marine sports photography. To that end, I never really needed a flash and didn't bother to learn about them. It wasn't until my first daughter was born and I dipped my toe into portrait photography that I realized what I'd been missing out on all those years. But flashes aren't just good for portrait photography.
And that brings us to this great video by Kai Wong, in which he walks you through how to use an on-camera flash and off-camera flash together in order to freeze motion. It's a very helpful video to understand some basic rules of using flash outside and the conditions when a flash will be better than natural or ambient light. In his example, he shows you how using a camera without a flash at shutter speed of 1/8,000th of a second is still inferior to using a flash synced with another speedlight. If you've never used a flash outside (on camera and off camera) and you're curious about how to set it all up and get the most out of it, then give this video a look and let me know your thoughts.