Answer:
It is not so much the career that requires a dissertation as it is the level of the degree. In other words, a dissertation is typically required to complete a doctoral degree.
The dissertation is a lengthy, formal treatise, especially one written by a candidate for the doctoral degree at a university; a thesis. After completion of the work (usually through extensive research), the student must then defend it before a committee of faculty and possibly other notable committee members who work within the field the degree was taken in.