It was there for aesthetic purposes.

But why was the triangle there?

A sketch is something that actually informs the real drawing. This doesn't inform it in any way, it's just a triangle that's there that happens to line up with the "hand"

It didn't line up with anything, though. I don't understand how it benefits the drawing at all. It didn't go with the fingers at all. Calling it a sketch doesn't tell me how it helps.

Like, that isn't a sketch, though.

really useful sketch

I'd reference it If I wanted to draw a hand with its fingers chopped off