Sets make it easier to manage, archive, and search Files.
Some blocks use Sets; for instance, the Slideshow Block (discussed in Appendix: Types of Blocks). When adding slides to the Slideshow Block, you can select a whole set at once to be added to the slideshow, which is much easier than adding each image separately.
When do you create Sets?
When you upload files, you can assign them to Sets at that time, as shown in Adding Files.
You can also assign files to Sets later, after you’ve already uploaded them.