Who's Who on a Film Crew

Why are there so many different jobs on a movie crew?  What is one job that you learned about in this video that you'd never heard of.  Why might it seem interesting?

    There are so many different jobs on a movie crew because there are so many different things that need to be done that require vastly different skillsets in order to run smoothly. For instance, the director is handling all the creative decisions on set whilst the assistant director handles the logistics on set. Although one can possibly do both, it is a tremendous amount of work, which is why roles like the assistant director exists. These jobs focus in one area so that each person on the crew isn't going to be doing more than they can handle and can actually do their jobs efficiently. One job that I learned about in this video is the gaffer. A gaffer creates the lighting designs and oversees its setup which I knew was a role but never knew the title. It seems interesting as I haven't learned that much about lighting and I hope to learn more.