Full Idea
To say that there is at least one thing x such that Fx we need only use an existential quantifier, but to say that there are at least two things we need identity as well.
Gist of Idea
If we are to express that there at least two things, we need identity
Source
David Bostock (Intermediate Logic [1997], 8.1)
Book Reference
Bostock,David: 'Intermediate Logic' [OUP 1997], p.328
A Reaction
The only clear account I've found of why logic may need to be 'with identity'. Without it, you can only reason about one thing or all things. Presumably plural quantification no longer requires '='?