The semicolon is used in the text above, to cause an intermediate effect between the comma and the period. In this case, when he used a semicolon the author was indicating to the reader that it was necessary to pause, in reading, longer than the pause of a comma, but less long than the pause of a point. This is done because the sentence displays a partially complete thought, but it will present more arguments to be finalized.
In a more simplified way, we can say that the use of a semicolon in the text above was made to separate long coordinated sentences with the same syntactic function.
