It is unusual for a narrator to address the reader, but there are instances when the narrator does directly address the reader. For example, Charles Dickens often addressed his readers as "dear reader" and spoke directly to them. The narrator does this for several reasons. Two such reasons are to point out important themes and to directly characterize characters. I do think this is effective when it's use is limited.
There are often times when the author wants to make a thematic point very clear to the reader, so the author directly addresses the reader. When the narrator directly addresses the reader, the reader feels more intimiately a part of the story and this can be a very effective and powerful tool. When the narrator is always talking to the reader, the effect is different. Whe sparingly used, the author can call attention to what's important this way.
Another method authors use is direct characterization. Sometimes the author wants to share an inside joke with the reader behind the character's back. Either way, the narrator sometimes needs to talk directly to the reader to share some information about one of the characters. This too can be very effective, because it draws the reader's attention to what the author feels is most important.