This is just a quick video talking about keeping some water in a temporary turnout area like an arena or small field. It’s always just nice for them to have the option to have a drink if they want.
Something to consider when turning out horses is that you never know where they are in their day. They may be just about to go get a drink and you might think to take them out to go have some fun and run around and they’re thirsty. Making sure they have a bucket of water or something in the turnout area, alleviates any stress they might have, or just discomfort from being thirsty and is an easy thing to do.