With the first, a user can only READ and REPLY to a thread, but can not START a new thread in that forum or permission set.
The second, a user can not only READ a post and REPLY to it, they can also START a new post.
With the first, you can have a forum where only certain individuals, like ADMINS can start a thread, but you want users to be able to post their input, but NOT start a new thread. This then keeps them from doing that.