Most displays have an aspect ratio of 4:3 or 16:10. When watching a movie with an aspect ratio of 2.35, it will only take about 50% of the vertical space.
In most other players you can zoom in a little bit (a custom amount), partially trimming off some content at the left and right side, so that a larger area of your screen is used. Here's what I mean:
Ideally you'd be able to manually slide the pan scan from 0% (top, default) to 100% (bottom).
Note that this is NOT changing the aspect ratio, which is shown on the right: you don't want to stretch the movie out of proportions (circles should be circles, not eggs).