why we configure dispatcher at author instance?

You should always be configuring dispatcher at the publisher level - dispatcher in front of publisher instances is the standard approach - almost all the dispatcher documentation is geared towards how to configure dispatcher for a publisher instance. 

In addition to configuring dispatcher in front of you publishers you can optionally configure it in front of you author instances. This is not a replacement for dispatcher in front of publishers but an additional step to take to improve the performance of your author boxes. When you configure dispatcher in front of author you configure differently in than if it's configured in publish - you configure it so that all it caches are the static elements of the authoring UI - icons and background images, product level javascript and CSS that only changes when you upgrade the product - those sorts. This improves the performance of you author instance by reducing the number of hits to the actual AEM instance - they get served from Dispatcher all the. Generally you do configure any cache flushing for an author dispatcher instance because you want to only change with an upgrade.

The only thing you have to be careful of is that you don't cache any content - that will result in issues with editing and accessing content. The link above gives good instructions on what to cache. 

Again even when you do configure dispatcher for author you still need to do a standard configuration in front of your publish servers.