Solution folders are handy for organizing (and sometimes hiding) projects in complex solutions. For this usage, it makes sense that the Solution Folders aren't required to map to physical folders. Another important use is for solution level dependencies: external assemblies referenced by multiple projects, documentation, key files, and such.

The biggest reason I hear from developers for not putting these items in a solution folder is that the physical folder mapping is a mess. Most of these developers advocate for adding a "Dependencies" class library project and marking it to not build.

Let's just have a container for solution items that works like a normal project folder does when adding items. I call this a "Solution Items Folder".