When you want to show the comparison between categories A and B. Category comparison is used when you are comparing nominal data. For example names of people, departments or products. You are looking at only one moment in time. 

For example if “you would like to show …” 

… the total revenue generated by the members of your sales department over a year, you can use a barchart

… the outcome of a questionnaire that has answers as ‘like’ and ‘unline’, you can use a diverging stacked barchart.

… the year to date value of different KPI’s in relation to their target you can use a bulletgraph