It might be useful to offer people a way to search for content that is rendered in the page, rather than content that is only visible in developer tools.
The content is rendered on the page. For example, say someone has an email sign-up box. When the page renders, the box appears and it might list all the countries in the world, so that you can pick your country from the list. All those countries are rendered, available if you use the box. But if you ctrl-f search, you might not see that text even though it did render. Real case I looked into which prompted the tip of using developer tools.