dotCMS keeps old versions of assets and content. All of this data takes up space and in some cases might impact performance. (Note: dotCMS does now periodically delete old non-active Elasticsearch indexes.)
The Drop Old Asset Versions operation allows you to delete all assets older than a certain date. This operation will never delete any live or working content, no matter its age.
This operation is located under System → Maintenance → Tools.
Clean options are also available for the server File System.