It’s the relativistic velocity addition formula, which makes zero sense in non-relativistic scenarios, but it’s useful because it caps the velocity addition to just below c.
EDIT: I misread. Yeah, even if correct you can’t just write down a formula and use it without justifying what it does or where it comes from.
You’re absolutely right. I probably should have specified that in this context, that formula comes completely out of the blue and is not really appropriate, so I feel like in this case it warrants a bit of explanation.