Idea Summary
Containers and regions can be collapsible vertically, but not horizontally, like a navigation menu. That's fine when you have stacked regions, but otherwise not helpful.

Use Case
If you collapse a region vertically, the user sees a blank space where the region was, but the remainder of the page stays where it was. Users want to get this area out of the way they aren't using it, so they have more screen real estate for their work. This is especially true with a filter region.

Preferred Solution (Optional)
Allow regions to collapse horizontally, like navigation menus. You can easily restore them when you need them. Otherwise, the entire screen space is available for the application.