“What is a person?
Eh?... Well, a person is something like us, that looks like this, and has a face...
There are others, too?
Huh? Ah, yes, there are...
Really? Are they all like you? Do they have a face like yours?
We're not the same. If we had the same face, it would be disturbing.”
It's impressive how much a simplistic dialog says about the global conflict AND the episode in question.
-Faces are a clear symbol of individuality in Gurren-Lagann. Hence the heavily characterized faced machines and the vilification of the mugan.
-By proxy, declares lack of individuality and "fitting in" as something disturbing.
-Hence portrays Simon's inner conflict after Kamina's demise. Him wanting to showcase a face that wasn't his.
-Similarly, it states the route of Nia's character development. Despite her looks, she was always one of millions: a doll for Lord Genome, a messenger for the Anti-Spiral. It's also her purpose for the narrative to portray another route to how reciprocal "individualization" is, since both she and Simon become individuals only through the help of each other.