What is the role of the swim buoy/buoy bag for each swimmer?

The swim buoy/floating dry bag is necessity that each swimmer need one,so the market is big.
It is widely used all over the world,such as USA,UK,Spain,Germany,Switzerland,Netherlands,Singapore,and so on.
Also it has many advantage:
1.Always be visible 
Let the boat traffic know you are in the water
2.Have a rest if needed 
The buoy will fully support your weight
3.Keep your stuff dry & away from thieves
 Take all your important things along for a swim
4.Portable & Lightweight
Leightweight to bring along for swimming, kayaking

 in the open water.