Big Tyrannosaurus rex, why its small arms?

This 11 March 2019 video says about itself:

Why did T. rex have small arms? | Natural History Museum [in London, England]

Tyrannosaurus‘s tiny arms are often joked about. But were their surprisingly muscular limbs actually more useful than they’re given credit for?

Museum scientist Kieran Miles explains some of the theories for what these famous dinosaurs may have used their disproportionately small appendages for.