A parallel sets plot is a representation of various dimensions with relationships. Each dimension has a horizontal line, which divides into the number of categories of that dimension. The width of the bar denotes the higher value of that category. Each category from the first dimension goes to the categories of the second dimension, and so on.
Why do you need it?
Parallel sets are useful as they make ribbons of connection which are easy to spot when their width is high. Every deviation of that ribbon from a dimension to further dimensions is easy to understand insight. It looks attractive as well as saves time at the same time.
What kind of data you can visualize with it?
Assume you have a dataset in which we have categories of survived and dead people due to a ship sink that falls into the first dimension. The ribbons from both these categories move and deviate into male and female categories of the second dimension. It further divides the ribbon into an adult or a child, which are in the third dimension, and so on if the dimensions continue. This is how parallel sets are beneficial.