You can mix them into any search alongside tags. 'system predicates', which search metadata other than simple tags, show on any search page with an empty autocomplete input. Let's look at the tag autocomplete dropdown: Just open a new search page ( pages > new file search page or Ctrl+T > file search) and start typing in the search field which should be focused when you first open the page.
The primary purpose of tags is to be able to find what you've tagged again.
