Anchor text of inbound links is for some years an important criterion of the Google algorithm. How a link to a page is a vote for that page, so is the anchor text a vote for a keyword for that page. This is the main reason why you can find Wikipedia on the top of many searches for competitive keywords in Google.

The importance of pagerank has been downgraded dramatically. There are many examples of low pagerank pages outranking high pagerank pages for competitive keywords. Now to day is the keywords used in the anchor tag from links pointing to the target page more important than the links to the target page on its own. Remember that the pagerank that Google display in the toolbar is outdated and toolbar pagerank updates occurring at three-month intervals.

There are multiple examples of pages with top rankings who have inbound links from unrelated sites. But topical relevance can become increasingly more important in the future. However links from topic related sites result has a higher CTR than unrelated sites. More important is the keywords used in the anchor tag from links pointing to the page.