Here's an excerpt from the module page:
"
Introduction
cache_warmer is a drush command that hits a set of URIs of a drupal site based on the freshnessof the content.
The main purpose is to offer a complete setup for running a mostly cached drupal site without having to deal with expiration logic and instead use microcaching.
Dealing with the expiration logic is messy and inefficient it requires additional work, be it from the drupal side of things, be it from the external cache side.
Microcaching can be used with any type of site. Be it your portfolio or personal blog or a high traffic news site. You have to tweak the cache validity (or Time To Live: TTL) for your site traffic profile.
Although microcaching is particularly useful for the Nginx filesystem based cache it can be used with other caching systems like Varnish.
It can be used also for priming any type of external cache
Note that as is usual with drush although its tagged 7.x it works with both drupal 6 and drupal 7."