Resolution Usage Metrics
Resolutions of tasks can be set in Jira workflows in similar way to workflow statuses (the default resolution values are Done, Won't Do, Closed etc, but you can add custom ones too).
With fields, statuses and screens, it gives you a nice number of places where they're used within Jira which helps you see how important they are (e.g. if number is high don't delete it). However, resolutions don't give you any indication of their usage so if you wanted to tidy up your Jira instance you'd have to manually search for tasks by filtering them with JQL.