Maile Ohye of Google Search team has this to say on adding descriptive text to outbound external links in web pages:
"Descriptive anchor text (the visible text in a hyperlink) helps accurately inter-connect the web. It allows both users and Googlebot to better understand what they’re likely to find when following a link to another page. So if it’s not too much trouble, try making anchor text descriptive."
Further she stresses that adding relevant external links in web pages is beneficial both for search engines and your visitors because they provide your readers in-depth information about similar topics.
"Thoughtful outbound links also help your credibility because it shows that you’ve done your research and have expertise in the subject manner. You visitors may therefore want to come back for more analysis on future topics."