I would echo all previous answers, not to be repetitive and add one common reason from my specific experience: creating credibility in new field. When I did my MBA at Queen's University, at the time it was called MBA for Science and Technology. Language is key as it was meant for engineers transitioning to management.
With respect to my peers, a number of them were entering finance, marketing, operations and IT and coming from mechanical, environmental, computer, automotive fields to name a few. The MBA provided credibility to get into the management areas now which may not have not been attainable due to their pure engineering backgrounds.