It makes you visible to boaters, jet skis, kite boarders, paddlers, surfers or even landing float planes while you are swimming, lessening your chance of being accidentally run over.
A swimmers’ progress can be more easily monitored by others, either on the water during an event by lifeguards, or just from shore by those monitoring your safety. It is a lot easier to see a bright orange buoy than a mostly submerged swimmer in a black wetsuit;
It can be used as a floatation device when you need it in the case of cramps, dizziness, injury, fatigue or anxiety; and it can also be used to rescue others who are in any kind of distress, without risking being pulled under by a desperate swimmer (They can be easily and quickly unbuckled to share for this purpose);
A swimmer can be more easily found and rescued in a race if still attached to a personal swim buoy.
Some models can be used as a storage device that you can take along with you, which especially useful in a point to point swim. You can also keep your “stuff” safe while you’re swimming if you don’t want to leave your valuables on the beach or in your car– No more need to worry about where to put your keys!