This will add an option to the staging database where you can right click and "Find unused fields".
This will bring up a search dialog that shows all staging fields that is not used anywhere. So it is not part of any data movements, transformations, lookups, joins, add related, conditions etc etc.
The reason for this is that many projects will contain more fields than needed, and it can clear out a lot of time when executing projects to remove unused fields.
The option should also be available on DW, even though unused fields there might be intended.