What is a basic figure 8 most commonly used for?

The basic figure 8 knot is primarily recognized for its role as a stopper knot. It is designed to prevent the end of a rope from slipping through a device or out of a knot. When tied at the end of a rope, the figure 8 creates a bulkier and more secure terminus, ensuring that it doesn't slip through devices like belay devices or carabiners, which helps in various climbing and rescue situations.

While the figure 8 can be utilized in different contexts, its most critical function is to serve as a stopper knot. This position is vital in rescue operations, ensuring the safety of climbers and rescuers by securely holding the rope in place and preventing unintended slip-throughs, which could result in accidents or equipment failure.