These floating balls are used for everything from light bait to heavy sea fishing. If you are bottom fishing with worms or shrimp, the bait wants to fall to the bottom but many fish will not eat from the bottom. At these times you can mount a floating ball before the hook, this allows the ball to lift the worm or shrimp from the bottom and you can present your bait in the open water. This allows you to target fish that do not feed from the bottom, such as rainbow trout, brown trout and char.
In sea fishing, floating balls are used on the line to protect the top eye of your rod. When the ball is sliding, you thread the line through it before tying on the leader. Then the ball will always be in the surface and when the bait comes to the surface and you happen to crank too far, the ball takes in the top loop and not the spawning. This means that you protect the ceramic in the top loop from breaking. If the loop breaks in the middle of the sea, the fishing day may be over and you don't want that, do you?