Why do doctors have multiple jobs/positions?

I see this all the time. One of the doctors I shadowed was in clinic only two days a week, then he was an associate professor at the nearby med school, and he was also running a dialysis clinic on the side. One of the doctors I work with is an assistant clinical professor and also works in our clinic 4 days a week. My dad’s friend is a hospitalist that also provides outpatient services. Why do so many doctors service multiple roles and even have multiple jobs? Surely it’s not about... Why do doctors have multiple jobs/positions?
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Tags: Pre-Medical - MD Source Type: forums