What's the difference between a side-by-side and a stacked (vertical) dual monitor arm?

Side-by-side arms position both monitors horizontally next to each other, which is the most common configuration for productivity. Vertical stacking arms (like the Ergotron LX Vertical Stacking) place one monitor above the other on a tall pole, which reviewers on r/Ergotron and r/ultrawidemasterrace prefer when desk width is limited or one screen is used for reference content above a primary monitor.