Use 'on behalf of somebody' when someone speaks for another who can't speak themselves at that moment.

Substitute Speaker

Someone must give you the right to act on their behalf, it's not taken by default.

Authority Given

Actions taken 'on behalf of somebody' can transfer responsibility to that person.

The phrase underlines who benefits from the action, not who performs it.

Actions 'on behalf of' someone often imply a selfless or supportive motive.

Using 'on behalf of somebody' indicates you're an advocate for their interests.