Many doctors like to do some form of non-clinical work because it adds a stream of income especially while their own private practice is getting started. I did life insurance exams for a few years, going into the beautiful homes of executives (and even other doctors) in the morning hours before working at my office (blood tests need to be fasting… therefore it was usually a morning activity). It was steady money, paid reliably to the penny at the end of every month. No denied payments, coding error penalties, etc.!
There are of course other reasons doctors like to engage in non-clinical activities. Many are very interesting and add a dimension to their career they might otherwise miss. More than a few doctors are successful authors, for instance. Their practice and all the interesting people and advisors they interact with each day are fertile material for their stories! Many doctors develop whole new careers to pursue after they leave clinical medicine from their non-clinical activities during their clinical career.