Or it’s a joke that the kit they supply, can be used with or without the arm. The arm is free, but be sure not to lose it, if you don’t use it. Someone might well want to use it, when you come to sell it on.
Or that it’s £7.99 part not necessary.
Same as they supply lever heads in everything when if you’re buying the maglock/wireless heads it’s yet something else not needed and going to waste.