Many engineers that decide to get a Doctoral degree choose something other than engineering, mainly because the options are so limited for Ph.D in engineering degrees, but also because it helps them diversify their skillset for the business world.
Many opt for a Doctorate in Project Management or a Doctorate in Business Administration, which helps them continue to advance, sometimes to executive levels, within their company. A PhD in Engineering or Project Management helps give engineers perspective on how a complete project works regardless of the department or company.