The $.speed() method provides a way to define properties, such as duration, easing, and queue, to use in a custom animation. By using it, you don't have to implement the logic that deals with default values and optional parameters.
This method is meant for plugin developers who are creating new animation methods. Letting $.speed() do all the parameter hockey and normalization for you, rather than duplicating the logic yourself, makes your work simpler. An example of use can be found in the animated form of .addClass() of jQuery UI.