It’s just another option, don’t gotta use it. Maybe you find yourself needing something like this, and the only other choice is making it public. At least with friend classes, you know which classes are friends so you can go look for any dependencies

There's infinite ways to organize code. In C# or Rust where this isn't an option, you might use nested classes or traits hidden behind a module/namespace.

Good use cases are data structures with associated helper classes. For example, a collection/tree and an iterator/tree-walker for working with elements of the collection. Or for something like a smart memory allocator (an arena or slab allocator), you might use a friend-class to wrap elements returned from the allocator, representing their connection back to it (for freeing up when done or to manage the allocation structure in ie a heap or sorted tree).