Configuring Workflow rules makes it easier to manage sales, marketing or support related activities by automating the entire process. While configuring a rule, you define a specific set of conditions based on which either a rule is applied or its not applied on the records. Most certainly, before configuring a rule you will have a vague idea about the number of records that can satisfy the criteria you have mentioned. But in order to get concrete insight on the rule's performance, you should know the number of records that pass through each of these configuration levels to decide if the execution criteria works effectively.
The workflow usage report is designed to give you record counts at each configuration stage in a workflow rule. For example, you can view the number of records that qualify a particular trigger, the total number of records that satisfy a given condition, record count for an instant or scheduled actions.
Consider that you have configured a rule to 'send emails' to the prospects who have recently attended a conference. The rule is triggered and the emails are sent but you have no idea whether all the prospects have received the email, whether it bounced, how many of them clicked on the email. With workflow usage report you can see the exact number of emails that were sent, unsent, bounced, unopened, clicked etc. and take respective actions.