Google has recently removed the related searches feature because few people used it. Don't confuse this feature with the regular related searches that are displayed below the search results. "We weren't seeing enough usage of this filter to maintain it in the toolbar, but we do display related searches at the bottom of the page when they seem relevant enough," explains Google.
Here's a screenshot from 2009 (licensed as Creative Commons by Search Engine Land):
"Related searches" allowed you to quickly check the results for similar queries. "If you're ever unsure about the precise terms you should use for your search, start out with a broader term then use the related searches tool to discover alternative search terms," explained Google. It was also used by SEOs to generated lists of related keywords.
The Google blog post from 2009 included the following video which shows a lot of features that are no longer available (review search, sentiment analysis snippets, longer snippets, wonder wheel, timeline, dictionary auto-linking, SearchWiki):