Can you share your ideas on
What does really use of controller in magento 2.
Well AKAIK - in Magento 2 controller is mainly used for routing purpose - meaning to route the actual path and call the specific methods !
Controllers are responsible for processing all requests made via a url. If you hit the url yourwebsitedomain.com/index.php/mycustom/test - so in the controllers/mycustom/test.php and inside of that the viewAction() method called
Refer this link for the same - https://devdocs.magento.com/guides/v2.2/extension-dev-guide/routing.html