Tags should be added to assets, templates and publications to help you and your colleagues easily find them in the future.
Add tags to your assets to ensure you will find it whenever you need it and search for it. Tags can be added in all kind of sorts. Add the name of the campaign it is used for, the name of the people on the picture, the name of the company department, a specific id or tag the name of a participating third party who helped creating your assets. Let your creativity run free, but make sure you structure your tags well and maintain consistency.