Maximum Dimension Stack will set the maximum width and/or height of a Stack. So, anything inside will not expand any further than this. This is very interesting for responsive themes where you don't want something to expand the whole size because its layout only works fine up to a specific width.
Take a look a the example of the light boxed text and image here. Example IV shows an image and a corresponding text Stack. On small screens everything is scaling fine. But as soon you are using desktop browsers the text is expanding very far on the x-axis. The image, which width is a maximum of 600px, will not expand any more. You could provide a bigger image, so that it will fill the whole lightbox. But if you want to show only small previews on purpose, this is not an option and you should limit the dimensions.