Full Idea
The empty set is said to be useful to express non-existence, but saying 'there are no Us', or ¬∃xUx are no less concise, and certainly less roundabout.
Gist of Idea
We don't need the empty set to express non-existence, as there are other ways to do that
Source
Oliver,A/Smiley,T (What are Sets and What are they For? [2006], 1.2)
Book Reference
'Metaphysics (Philosophical Perspectives 20)', ed/tr. Hawthorne,John [Blackwell 2006], p.130