The desktop is a convenient place to save files. You can get to the file you need quickly; it’s right there on the desktop, and you don’t need to sift through folders. Problem is, the desktop can get very cluttered, and your access efficiency diminishes every time you save a file to the desktop without removing one.
The new desktop Stacks feature in macOS Mojave helps organize the clutter. (It’s actually not a brand-new feature; Stacks made its first appearance in the OS X Dock. You can choose to have a group of items in the Dock appear as a Folder or as a Stack. The desktop implementation is new, however.) As the name implies, Stacks puts your desktop files into stacks. When you click on a stack, it expands to show the individual files. You can also scrub through a stack of files to find the one you need.