Contrary to vincaslt's comment, I believe n is necessary in order to catch the condition vectorjohn points out.

In this case it not only copies the array so it isn't modified (a best practice) but also handles when n is less than 3. Without it he'd return the whole signature.

It just creates a copy of an array, so that actual argument is not mutated. n could be ommited here, without change, making this less confusing.