If the sentence has too many commas, it becomes very confusing. The semicolon is a combination of a colon and a comma, and breaks the sentence up so that it's easier for the reader to understand.
Explanation: Semi-colons allow the sentence to be broken up into spaces so that it is understandable. Without using too many commas. Although, if a semi-colon or a comma is used too much it can still create a run-on sentence.
