The float is used to enable the angler to cast out a bait away from the shore or boat while maintaining a reference point to where the bait is unlike bottom or leger fishing.

Whenever you are float fishing you should always try to fish as light as you can.
Fishing light means using a float that needs the least amount of shot but don't fish too light if conditions, such as the weather, make it awkward and difficult to cast or control your float. In this situation it is better to have a heavier, properly shot float.