Full Idea
Thomason says with no unit sets we couldn't call {1,2}∩{2,3} a set - but so what? Why shouldn't the intersection be the number 2? However, we then have to distinguish three different cases of intersection (common subset or member, or disjoint).
Gist of Idea
The unit set may be needed to express intersections that leave a single member
Source
Oliver,A/Smiley,T (What are Sets and What are they For? [2006], 2.2)
Book Reference
'Metaphysics (Philosophical Perspectives 20)', ed/tr. Hawthorne,John [Blackwell 2006], p.135