If we’re already crawling your site, we might as well update your cached pages in the meanwhile!
The idea is as follows: if you’ve enabled our broken links or mixed content checks for any of your sites, we’ll crawl your sites to find any broken pages.
On top of that, we have the ability to set custom HTTP headers per website that get added to both the uptime checks and our crawler.
Combining our crawler and the custom HTTP headers allows you to authorize our crawler in your Varnish configs to let it update the cache.
Source: Using Oh Dear! to keep your Varnish cache warm – Oh Dear! blog