A built-in Revit command that finds and removes elements loaded into a project but not actually placed anywhere - unused family types, line styles, materials, view templates - which quietly bloat file size and slow down performance over the life of a project without adding any value.

Purge Unused is one of the simplest, lowest-risk habits for keeping a model healthy: run it periodically (especially before a big milestone or handover), and a project that has picked up hundreds of unused, half-experimented-with elements over months of work gets noticeably lighter and faster with essentially no downside.