[mitchs]

The important part is it safely allows you to pass references or other objects with lifetimes to those threads. (Or at least lets the compiler know so you can avoid having to write unsafe things yourself.)

This happened to be useful for something I was doing at work where the wrapper for a native C library expressed the required lifetimes of the library constructs using lifetimes. (Eg. You must set up Foo before Bar, and Foo must outlive Bar. It was OK to pass Bar and use it from multiple threads, but it had a lifetime attached. Using scoped threads we can satisfy the compiler's lifetime checker, since it knows even though we gave Bar to many threads, none of them outlive Foo.)