It starts with a vision. You can see something unfolding out there in the future that for some reason, other people can't.
Steve Jobs saw computers as tools for artists and creative people. Phil Knight saw technology used to unlock our potential as athletes. Howard Schultz saw coffee culture taking root in America. Martin King saw little white boys and girls and black boys and girls playing as sisters and brothers.
From that vision - either your own or someone else's you share and support - purpose flows. Your work, unhesitatingly, is about contributing to the fruition of the vision. Doing what it takes to make it come true.
Does a vision have to serve people? Yes. And the loftier, more expansive and inclusive the vision, the more potential it has to grow into a substantial tree. One that lots of people can fit under.
Vision and purpose are internal affairs, best experienced and understood by the world through your actions, understood as a story, supported and fed by roots - your history and foundation in the world.
What is my vision? What am I doing to accomplish it?