The Figure 8 Knot provides a quick and convenient stopper knot to prevent a line from sliding out of sight, e.g., up inside the mast. Its virtue is that, even after it has been jammed tightly against a block, it doesn’t bind; it can be undone easily. This virtue is also, occasionally, a vice. The Figure 8 Knot can come undone and then must be retied.
For climbing, where safety is paramount, the Double Overhand Knot is the preferred Stopper knot. However, the Figure 8 Knot is important to climbers because it is the basis for tying the Figure 8 Bend (Rope Join), the Figure 8 Loop Follow Through, and the Double Figure 8 Loop.
The Cow Hitch is used to secure a rope to a post or other object, including another rope. It is similar to a Clove Hitch except that the second half hitch is in the opposite direction.