Understanding it
noindex suits pages useful to visitors but pointless in Google: thank-you pages after a form, test areas, internal search results, ad landing pages that duplicate existing pages.
Google is clear: the rule cannot be declared in robots.txt. And if robots.txt blocks the page, Google never reads the noindex.
A noindex page can still get visitors through a link, an ad or an email: it only disappears from search results.
A concrete example
Illustrative example: ad landing pages
An agency builds pages dedicated to its Google Ads campaigns, close to its service pages. It does not want them competing in organic results.
The landing pages carry a noindex tag. They take the paid traffic, while the service pages keep their place in Google alone.
The common mistake
Combining noindex with a robots.txt block
Blocking a page in robots.txt and adding noindex stops Google from reading the rule: the page may stay listed, with no description. To exclude a page, keep it crawlable with a noindex.