It's useful if you want to pick only some sprites and not a whole spritesheet.
For instance with Godot you can use a spritesheet for a tilemap with multiple layers and have some standalone, reusable, scenes with their own asset i.e. a single file included in their directory.
Viewing post in 144th update: Theme Sorter + Single Files 3/7 comments
There are several uses for the single named files, but the one that convinced me to start this tedious work was the fact that singles seem to be super handy in Unity, since slicing up my tilesets isn't easy and they are quite big, so you tend to include a lot of stuff that isn't even gonna be used in the project!
I can't wait to start drawing new sprites tho, this is super boring