Think of it as enforced documentation. If I say that my object is a Customer then I know what can be done with it. If I have a method that says it takes a Cus

Think of it as enforced documentation. If I say that my object is a Customer then I know what can be done with it. If I have a method that says it takes a Customer object then the calling point knows what to pass in, and my method knows it can rely on that object satisfying certain conditions. Basically it removes (some of) the need to document separately, and it enforces that the documentation is adhered to.