Why overload operator() in a struct

Question

I have read some boost::thread code and found a lot of overloads of operator() in a struct. For instance, in boost/thread/thread_functors.hpp, you can see something like this:

  struct detach
  {
    void operator()(thread& t)
    {
      t.detach();
    }
  };

I don't understand what the benefits are. Why not use functions like

void detach(thread& t) {...}

instead?