Why would you work in ergonomics? Why would you work in Usability?
The simple answer to both questions is 'to help people'. Ergonomics does so by making the world safer and its workplaces less hazardous. Usability does so by making the world more intuitive, understandable and functional. Both focus on satisfying people.
However, there are many other goals. Some human factors professionals try to increase productivity; others seek long term cost-savings, while others still may try to tailor products to be universally accessible.