> But suppose your purpose in life is to save millions of lives, like what Bill Gates has done with his foundation
It could just as likely be the other way around: the reason Bill Gates created the foundation was to make him have a purpose in life after leaving Microsoft. If he hadn't created the foundation, he would now just be an extremly rich nobody. Saving lives is just a nice side-effect. Power is just a tool in this case, not the goal.
Originally, Bill Gates had neither. He used the power gained by fufulling his purpose of writing software to find a higher purpose. Id imaging his sense of purpose changed as his power (and resulting responsibility!) changed.
It could just as likely be the other way around: the reason Bill Gates created the foundation was to make him have a purpose in life after leaving Microsoft. If he hadn't created the foundation, he would now just be an extremly rich nobody. Saving lives is just a nice side-effect. Power is just a tool in this case, not the goal.