Links to external websites can help a site target its keywords and customers. External links also will cause a site to lose some of its PageRank, so there is an important balance that must be considered when putting external links onto a website.
External links help to create keywords on your site that would not otherwise exists. Perhaps your site does not contain any content on items that you would like on your keyword list. Find other sites on the internet that do offer this content, and would be beneficial for your users, and link to them. Make sure that the words you use for the link contain the keywords that you want to promote.
When you link to an external site, the portion of the linking page’s PageRank is lost to another site. This loss of PageRank forces the search engine conscious to remove any links that do not provide needed keywords for the site. All links to external sites should be carefully considered.