The big thing is that you need a P.E. in many cases to do things like sign off on drawings for regulators. Some civil engineers, mechanical engineers, etc. have PE's and many don't. In Louisiana, I had business cards with an engineering title and definitely worked as an engineer. At some point, had I remained in the oil business, I'd have gotten a PE because I'd presumably have eventually been in a position where I had approval authority over designs submitted to various government agencies.