We’re talking a simple enough sandwich that’s not stacked so high that it has to be cut.
I would say more times than not, I cut it in half. Usually right down the middle but if in a wild and crazy mood, I might go diagonally.
As to why: I dunno. I think it’s one of those “it’s what I’ve always done” reasons or simply, “because.” And while it might contribute to the sandwich’s aesthete, that can only last but a couple of seconds before total consumption is underway.