Speak of cleaning, it can be a real struggle to get a decanter properly cleaned. Rinsing with plenty of warm water helps, as does drying on a rack upside down. You can buy those specialized drying racks by the way, though I've always been able to manage to get my decanter to stay upside down while it dried without one.
Back to the balls though. Even doing all that, I have found that some residue does tend to build up on the bottom of my decanters, especially when I leave them out overnight with a touch of wine left in them. These decanter washing balls are the solution. They agitate the residue left in the decanter without effecting the decanter and obviate the need for harsh chemicals, which may leave a residue and affect the flavor and aroma of your next wine. They also make for great stocking stuffers!