The web was built on links, the modern web is all about relevance and reputation, relevance to the user's interests, and relevance to the content on the page.
In journalistic terms it's a way of showing the workings of a journalist, backing up facts, showing sources, leading the reader on to their own points of interest.
All this enhances the originating website's reputation and the visitors trust of the publisher/brand. Leaving articles as dead ends can leave unanswered questions for readers, there's always plenty of other content online, so losing your audience's interest is like showing them the door.
As The SEO Company's study shows:
"a good number of online news websites are very good at crediting sources with the appropriate link... Offering useful links actually makes visitors more likely to return to see what other interesting websites they might find in the future, a model that sites such as Digg and Fark are built around."
In terms of search engine optimisation, it's great for the Google ranking of a page to link out to related, reliable and trusted sources.