but not all bounty managers need this method. Every project has its own rules, if Mnager Bounty doesn't need Proof of Authentication then you don't have to. It's simple, if you don't want to get a negative Trust or be perceived as spam, then you should carefully read every project rule you want to follow.
I rarely seen the managers who didn't put the POA but they will be using the address as the verification to make sure if that has owned by the real user.
Even the bitcoin campaign needs POA to make sure if the manager can clarify the authentication of the its participants.
It's not all but almost all of managers are using this method.