Is Drupal Cache Warmer for real? I am using  Varnish + Default Page cache with a expiring time (1 day) in a site.

When I am browsing the site, everything seems loading fine; once I come back to the site after a day or so, the the pages start loading slow and they become okay after a while.

It seems like the cache is being cleared after a while and generated again once someone visit the site again. I do not want that user to suffer. 

I read about Drupal Cache Warmer; is there something which can keep the cache alive and no user suffers?
Well, there's the Cache Warmer module you can set up if you have drush access on your server.

You can also use a 3rd party service like Pingdom to do that as well.