Why do some people use swap for move assignments?

For example, stdlibc++ has the following:

unique_lock& operator=(unique_lock&& __u)
{
    if(_M_owns)
        unlock();
    unique_lock(std::move(__u)).swap(*this);
    __u._M_device = 0;
    __u._M_owns = false;
    return *this;
}

Why not just assign the two __u members to *this directly? Doesn't the swap imply that __u is assigned the *this members, only to later have then assigned 0 and false... in which case the swap is doing unnecessary work. What am I missing? (the unique_lock::swap just does an std::swap on each member)