With linear types you can guarantee that a destructor is run, so you can create objects with lifetimes that are not only bounded from below, but bounded by above. There are some common patterns in e.g. C libraries that rely on this--for example, you might register a callback on an object, and you want to assert that the object's lifetime is shorter than the callback's (so the callback is never called outside its lifetime).
Since Rust doesn't have linear types, you have to use unsafe{}.
