Review the item:
Although you might not believe it,
Antonio actually benefits from his chicken scratch handwriting because
no one can read it, Antonio does not worry about word choice or spelling.
Although you might not believe it is a subordinate clause. A subordinate clause cannot stand alone; it must be connected to a main clause. Antonio actually benefits from his chicken scratch handwriting is the necessary main clause.
When a subordinate clause introduces a main clause, as in the example above, you must put a comma between the two.