what benefit is it to clearing the Drupal cache now that I have Varnish in front of Drupal?
Whenever Varnish Cache expires it asks Drupal to give it the rendered page. Drupal in turn sees if it can server the page from the Cache. If yes it will server from the cache, else it will regenerate the page. 

In short, when you clear the Drupal Cache, 


No changes are visible to end users for whom the content is served from Varnish.
When the Varnish Cache expires, it will fetch the content form Drupal. Now that you had cleared the Drupal cache, instead of serving from the cache the page will be rebuilt.