It has some purpose under different scenarios. Let me explain:
Suppose, you have 10 post forms on your site and you are taking post using all of them. Well, you got a post which is available on the wp-admin->posts, you have opened it. Now, how could you understand which form has been used to submit this post? Yes, the form will be shown as selected on this post meta box. 🙂
Also, think you have an older post which doesn't have any custom field. Consider a default WordPress post. Now, you want to have the custom fields with the submitted post that was available with a form. In that case, if you select that certain form while editing the post, it will get the available custom fields.