False casting is simply the classic back and forth fly casting motion everyone has seen. It is necessary to false cast when fly fishing because we’re casting a long, weighted line instead of a simple weight, bait or lure as used in spin or bait casting. False casting allows us to change cast distance, change direction, and dry a wet fly. Learning how to false cast is an important step in the process of learning to cast. I often use a false cast or two to measure distance. If I see a rising trout that I estimate to be 40 ' away, I will false cast to an area away from the fish to make sure my fly is at the right distane to land that fly just a few feet above the rising trout. It is most useful if there are other smaller fish between you and your target and you would rather not hook them.

When and WHY do you false cast in fly fishing? by Red's Fly Shop