What is the main use of a life ring on a vessel?

A life ring, also known as a lifebuoy, is primarily used to provide buoyancy for individuals who have fallen overboard. Its design allows it to be easily tossed to someone in the water, providing immediate flotation support and helping to keep them afloat until they can be rescued. This is crucial in emergency situations where every second counts, and the life ring becomes an essential safety device designed specifically for this purpose.

While the other options mention various useful functionalities, they do not align with the primary purpose of a life ring. For instance, aiding in swimming practice is not a role the life ring fulfills, as it is not intended to be used in training scenarios. Assisting in towing lifeboats is also not a function of the life ring; instead, specialized gear is used for towing. Lastly, signaling for help typically involves other equipment, such as flares or whistles, rather than the life ring, which is focused on providing immediate flotation support.