It allows you to look for specific patterns on headers, links and other hard to search parts of sites.
For example, you can search for all sites using PHP/5.2:
https://dnsarchive.net/web-search?q=PHP/5.2
Or WordPress 5.2:
https://dnsarchive.net/web-search?q=%22Wordpress%205.2%22
And things like that. Feedback welcome!