A swim lane diagram is a visual representation of a system that includes all the steps, processes, and actors within it. Instead of describing a system with words, a swim lane diagram can allow for a much easier explanation. The shapes used in a swim lane diagram are circles to signify the beginning and end of the system, arrows to indicate the direction of the steps, rectangles to indicate steps in the process, and diamonds to indicate decisions that must be made by actors. Swim lane diagrams allow the order of a system to be easily understood as opposed to reading an explanation of it.
Swim lane diagrams are definitely good for making processes easier to visualize and understand. I think they especially benefit the actors involved in the process to see what their role is.