It's fun.
Blue Beetle's wonderful, wonderful scarab-robot-suit thing is rather vicious too.
It insists upon killing many things.
So Jamie is constantly telling it that "No, We are not shooting the cashier! She's just asking if we want paper or plastic!"
And since no one else can hear it talk... well..
Everyone scootches away slowly.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather