Allocator usage in C++ (STL Tree)

I've recently been trying to understand how c++ allocators work, and I've been looking to the implementation of the red-black tree that the STL library uses for things like std::set or std::map, but there are some things that I can't get my head around.

The first thing that does is convert the allocator from the type the container has to store - _Val - to the type of the node that the tree uses - _Rb_tree_node<_Val> - using the rebind template:

typedef typename __gnu_cxx::__alloc_traits<_Alloc>::template
    rebind<_Rb_tree_node<_Val> >::other _Node_allocator;

typedef __gnu_cxx::__alloc_traits<_Node_allocator> _Alloc_traits;

This I can sort out.

Now, when an element is inserted and it needs to create a new node what it does is this

_Node_type __node = _Alloc_traits::allocate(_M_get_Node_allocator(), 1);

which I assume allocates space for a single node. But then it does this

::new(__node) _Rb_tree_node<_Val>;

which I really don't know what it does, since the space for __node has already been allocated. But after that it also does this

_Alloc_traits::construct(_M_get_Node_allocator(), __node->_M_valptr(), ...);

which makes me even more confused, because is supposedly constructing a node (is the node allocator), but it passes the pointer __node->_M_valptr() which is of type _Val*.

If someone could explain this, I would be very grateful.