Suppose that we have a web page. This page could be vulnerable to any attack such as SQLI. By looking the HTML code of the page, we can find some input elements that could be used by attackers to exploit vulnerabilities.
So we can guess that pages which have more input elements (for example), might be more vulnerable. I mean these pages might be more risky. So I want to conclude that these pages are more important for security scanners, and testing these pages may result in finding more vulnerabilities.
So one of the factors to decide that a web page is more important (in security manner) than the other page, is the number of input elements in the page. Another factor may be number of parameters we could pass to the page URL.