it helps keep the rope out of the water, and makes it visible to other boaters who are not paying attention. I could use one on my starflite, but just haven't purchased one yet.
Well, close.
On a lot of outboard boats, the Y-harness can sit extremely low to the water. This causes the tow rope to cut into the wake when tubing, as the tow point on the tube is also extemely low. When the wake catches the tow rope, it actually will cause the tube to spin violently 'in' at a 90 degree angle to the boat.
When the rope then comes out of the wake, it releases and snaps the tube back violently. Kids were scared to ride in tubes after a couple of incidents, so we got this.
This ball raises the rope up just enough to keep everything out of the wake...works great! Had it 3 years now and its been worth the money.
Its also supposed to be a shock absorber. In the photo you can see its compressed, normally, its not, so I think it does absorb some shock in the line.