Flow router: this.autorun vs subscription in router

It may seem a silly question, but as I understand flow router is not reactive, but when I try to do this

subscriptions: function() {
   this.register('posts', Meteor.subscribe('posts'));

and on clien

{{#each posts}}

All works reactive, when I insert new post from client (or I not understand what is reactive ??)
So when I read this subscription in Template I dont understand why I must use autorun when I can use subscription form flow router ?

The reasoning is explained here: https://kadira.io/academy/meteor-routing-guide/content/subscriptions-and-data-management

too abstract, I see no reason to change the ease router sub to autorun in helpers

EDIT : after this , I see why