Semi-colons can be used to join 2 complete sentences. A comma isn't strong enough to hold 2 sentences together by itself, so it needs an extra dot of glue. That's a ;
EX:
John was afraid of water. He never wanted to go sailing.
John was afraid of water; he never wanted to go sailing.
Why would you do this?
Writers need to use different types of sentences to make the writing more interesting. This is just one more way to join two small sentences to create a different type of sentence.