^^ says "focus my target if it can be attacked *and* if it exists". If your target can be attacked (if [harm] is true), it exists. The exists condition does nothing for you in this case - it is implicitly implied with the harm condition.

It's not hurting you. But it's not helping you, either. ;)


/edit: if you have no target, then [harm] will return false (because you can't attack a target that does not exist), and /focus will not be called.