On the Purdue OWL website, http://owl.english.purdue.edu/engagement/index.php?category_id=2&sub_category_id=1&article_id=44 it states that a semi-colon can be used for joining two independent clauses. These two clauses are related to one another, when the semi-colon is used.
The other reason a semi-colon could be used would be to use semi-colons in between items that already have commas. For example, "I have been to Albany, New York; Cleveland, Ohio; Miami, Florida." In the above image, there are no commas for a semi-colon to even be placed.