It was done for exactly one reason: To be different.

Whenever video and tv got a certain aspect ratio, film "one upped" it. Part of it was also the size of 35mm film, once they moved the audio track off of the central area it made it wider, so they used that area.

Basically, there is no fundamental reason for it. But these days people are used to it and think it was done because video somehow requires it.

Now that they make lots of money from selling home video they will probably not move too far from 16/9, but it wasn't always so.

It's the same with the poor FPS of film: It makes it look like a "film". Same for graininess. It's not better, people just associate it with movies.