What is the figure eight bend knot used for?

The figure eight bend knot is specifically designed to join two ropes securely, making it an ideal choice for connecting two lines together to create a longer line. This type of knot is advantageous because it is strong, easy to tie, and provides a reliable connection that can be untied easily under tension.

In the context of the other options, while knots serve many purposes in boating and sailing, this particular knot is not typically used for tying a boat to a dock or securing a sail. Additionally, it is not suitable for attaching an anchor to a boat, as other knots, like the bowline or a clove hitch, are favored for those specific situations due to their characteristics and reliability under the loads they encounter.