Even if Google shows a link to this feature next to each Google result, few people click on "similar" to find related web pages. That's probably the reason why Google decided to show at the bottom of the search results page an OneBox with pages that are related to the top result, but only if the pages are likely to be helpful and only for navigational queries.
"Since we've been continuously improving this feature and we think it's really useful, we're now going to start showing these alternative sites more prominently," explains Google.
The feature is only available in the US and it's an alternate way to show a list of related searches. Instead of showing similar searches, Google displays a list of pages that are similar to the top result.
If you're wondering how Google manages to find related pages, read this article.
