Search engines need to decide what pages to include in their index. Discovering pages by crawling the Web (following links) is one way they discover web pages (the other is through the use of XML Sitemap files). In addition, the search engines do not include pages that they deem to be of low value because cluttering their index with those pages will not lead to a good experience for their users. The cumulative link value, or link juice, of a page is a factor in making that decision.
Search engine spiders go out and crawl a portion of the Web every day. This is no small task, and it starts with deciding where to begin and where to go. Google has publicly indicated that it starts its crawl in PageRank order. In other words, it crawls PageRank 10 sites first, PageRank 9 sites next, and so on. Higher PageRank sites also get crawled more deeply than other sites. It is likely that other search engines start their crawl with the most important sites first as well. This would make sense, because changes on the most important sites are the ones the search engines want to discover first. In addition, if a very important site links to a new resource for the first time, the search engines tend to place a lot of trust in that link and want to factor the new link (vote) into their algorithms quickly.
Links play a critical role in ranking. For example, consider two sites where the on-page content is equally relevant to a given topic. Perhaps they are the shopping sites Amazon.com and (the less popular) JoesShoppingSite.com. The search engine needs a way to decide who comes out on top: Amazon or Joe. This is where links come in. Links cast the deciding vote. If more sites, and more important sites,
link to it, it must be more important, so Amazon wins.