Sometimes your just need a buffer for a syscall. Newing a buffer is canonical and unique_ptr isn’t free enough to use in the most performance sensitive contexts.
I worked with a team which used raw pointers instead of unique_ptr as they use more memory (twice as much, I think). The core data structure was a tree, and the tree was several hundred GB in size. So they were doing whatever they could to shave off memory.
Unique pointers imply ownership of the underlying memory. If you have multiple references to the same data, or in your case a tree, it might make sense to use raw pointers (or references) to access the tree data. It would not make sense to grant ownership of the same memory to two unique_ptr, or make copies of data unnecessarily.
In that case, you'd have something like `std::vector` or `std::unique_ptr` which owns the actual allocation (and governs the scoping and therefore deallocation), while using `vector.data()` or `unique_ptr.get()` to pass around an unowned pointer. All raw pointers can then be considered to be unowned by convention.