I don't get it when people say that w/ out explaining further.
It's a visual art concept. The movie Pacific Rim was all about tall and not so much wide. IMAX AR with it's additional height is preferred.
If the movie was about wide open spaces then the visual of lots of tall into the sky has no importance.
People who don't shoot photography often don't get it. People who just shoot pictures, mostly don't understand photographic composition, such as leading space and 2/3 rule and such.
In the home theater there are two ways home theater designers create variable aspect ratio screens. One is where the height is fixed and side curtains reveal more width for scope ( 2.35) and another is where the width is fixed and a valence and a skirt reveals additional height. I have a custom VAR that I can do both. The use of side curtains is mainly for those upconverted TV programs that show with a smaller than HD screen size.
Hope that helps.