Expanding Knowledge - Linking out provides a means for your audience to expand their wealth of knowledge by picking up websites they may not have known about before. This aids in reading content from multiple perspectives. Also, providing links related to whatever niche you are writing about will show readers that you're an expert in the field and keep up with the latest and greatest for that particular niche. Consider this perceived expertise.

Networking - Linking out when publishing an article generally initiates what are known as Pingbacks or Trackbacks. These little notifications inform a blog owner that someone has linked to one of their articles. I can tell you based on experience that I've personally discovered some great sites by checking out my trackback/pingback notifications and it's also proven to be a great opportunity to network with other blog owners.